mDia1 (also known as Dia1, Drf1 for Diaphanous-related formin-1, Diaph1, KIAA4062, p140mDia, mKIAA4062, or D18Wsu154e) is a member of the protein family called the formins and is a

Rho Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician letter res . Its uppercase form uses the sa ...

effector. It is the mouse version of the diaphanous homolog 1 of Drosophila. mDia1 localizes to cells' mitotic spindle and midbody, plays a role in stress fiber and filopodia formation, phagocytosis, activation of serum response factor, formation of adherens junctions, and it can act as a transcription factor. mDia1 accelerates

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...

nucleation and elongation by interacting with barbed ends (fast-growing ends) of actin filaments. The gene encoding mDia1 is located on Chromosome 18 of Mus musculus and named ''Diap1''. mDia1 is highly homologous to Drosophila diaphanous, regulating the cytokinetic ring during cytokinesis. Homologues in other species are known as well, like the human DIAP1, budding yeast Bni1 or fission yeast Cdc12p. The gene has been knocked-out in mice.

Structure

The product of the ''Diap1'' (''Diaph1'') gene consists of 1255 amino acids resulting in a molecular weight of 139,343 daltons. The mDia1 polypeptide chain can be divided into four protein domains: * GBD/FH3 (

Rho GTPase The Rho family of GTPases is a family of small (~21 kDa) signaling G proteins, and is a subfamily of the Ras superfamily. The members of the Rho GTPase family have been shown to regulate many aspects of intracellular actin dynamics, and are found ...

-binding domain/formin homology 3)domain (366 amino acids long): positions 75-440 * FH1 (formin homology 1) domain (162 amino acids long): positions 586-747 * FH2 (formin homology 2) domain (403 amino acids long): positions 752-1154 * DAD (diaphanous autoregulatory domain) (29 amino acids long): positions 1177-1205 Three supplementary domains were discovered: * coiled coil (103 amino acids long): positions 460-562 * coiled coil (153 amino acids long): positions 1027-1179 * Arg/Lys-rich domain (4 amino acids long): positions: 1196-1199 The active region of the C terminus consists of formin homology 1 and 2 (FH1 and FH2) and the Dia autoregulatory domain (DAD). The FH1 domain is predicted to be rope-like and it contains binding sites for

profilin Profilin is an actin-binding protein involved in the dynamic turnover and reconstruction of the actin cytoskeleton. It is found in all eukaryotic organisms. Profilin is important for spatially and temporally controlled growth of actin microfilame ...

-actin complexes. The adjacent FH2 domain forms together with the FH2 domain of a second mDia1 molecule a head-to-tail doughnut shaped dimer that encircles the barbed end of an actin filament. Thus the FH2 domain has the ability to dimerize. The N terminus consists of a

-binding domain (GBD), which is joint to the formin homology 3 (FH3). DAD can mediate autoinhibition through interactions with the Dia inhibitory domain (DID), which is a subdomain of the GDB/FH3 domain (see section Regulation).

Regulation

Autoinhibition is achieved through binding of the C-terminal DAD to the N-terminal DID. This interaction inhibits the ability of FH2 to nucleate

assembly. Rho-GTP binds to the GDB domain and disrupts the DAD-DID-interaction thus promoting actin assembly. But this requires high concentrations of Rho-GTP, which may be not physiological. Hence, the release of mDia1 from autoinhibition seems to require nonspecific membrane-associated factors that cooperate with Rho-GTP. Several binding proteins can regulate mDia1 localization and activity: * ABI1: Helps to localize mDia1 to lamellipodia, filopodia and cell adhesions * CLIP 170: Binds the FH2 domain and recruits mDia1 to sites of phagocytosis * Gα12/13: Helps to localize mDia1 to the leading edge of migrating cells * RhoA: Required for mDia1 localization to adherens junctions and partially removes mDia's autoinhibition * RhoB: Helps to localize mDia1 to endosomes Furthermore the scaffold protein (

IQGAP1 Ras GTPase-activating-like protein IQGAP1 (IQGAP1) also known as p195 is a ubiquitously expressed protein that in humans is encoded by the ''IQGAP1'' gene. IQGAP1 is a scaffold protein involved in regulating various cellular processes ranging from ...

) seems to impact on mDia1. IQGAP1 regulates the localization of mDia1 to the leading edge to cells. The only tested short C-terminal fragment of IQGAP1 (aa 1503 to 1657) was not activating the mDia1

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

activity ''in vitro''. However expression of this fragment in macrophages reduced phagocytosis. Thus it remains open if IQGAP1 influences the activity of mDia1 directly like it does for NWASP.

Mechanism

Nucleation

In contrast to the Arp 2/3 complex,

formins Formins (formin homology proteins) are a group of proteins that are involved in the polymerization of actin and associate with the fast-growing end (barbed end) of actin filaments. Most formins are Rho-GTPase effector proteins. Formins reg ...

nucleate the formation of unbranched actin filaments. FH2 domains lack structural similarity to actin but can bind actin monomers with very weak affinity. The FH2 dimer nucleates filament assembly by interacting directly with and stabilizing actin polymerization intermediates (dimers and trimers).

Elongation

A formin

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ...

remains constantly bound to the plus end of an actin filament despite ongoing polymerization. One

formin Formins (formin homology proteins) are a group of proteins that are involved in the polymerization of actin and associate with the fast-growing end (barbed end) of actin filaments. Most formins are Rho-GTPase effector proteins. Formins regu ...

of a dimer dissociates from the barbed end to take the next step while the second formin of the dimer remains bound. Thus the formin dimer processively adds actin monomers to the barbed end and are constantly present at the barbed end of an actin filament (processive capping). The FH1 domain recruits actin monomers through

binding, but it does not promote nucleation. Studies demonstrated that FH2 domains protect the rapidly elongating barbed ends of filaments from the vast molar excesses of actin capping proteins. The precise mechanisms of

actin filament Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other pro ...

nucleation remains an area of active investigation. The rate of FH2 movement while elongation on an

matches the rate of actin subunit addition, which can exceed 100 subunits per second.

Profilin Profilin is an actin-binding protein involved in the dynamic turnover and reconstruction of the actin cytoskeleton. It is found in all eukaryotic organisms. Profilin is important for spatially and temporally controlled growth of actin microfilame ...

as a ubiquitous actin-binding protein is associated with most actin monomers in cells. Interactions between profilin-actin with the FH1 domain can accelerate the elongation at the FH2-capped barbed ends.

Function

The

homology protein mDia1 is a

GTPase GTPases are a large family of hydrolase enzymes that bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP). The GTP binding and hydrolysis takes place in the highly conserved P-loop "G domain", a pro ...

effector protein, which appears to be universally present in eukaryotic cells and participates in: *

Stress fiber Stress fibers are contractile actin bundles found in non-muscle cells. They are composed of actin (microfilaments) and non-muscle myosin II (NMMII), and also contain various crosslinking proteins, such as α-actinin, to form a highly regulated ...

formation * Endocytosis * Microtubule functions

s are acto-myosin structures, which are important for establishment of cellular tension and thus traction to move a cell ahead; the later is mediated via cell adhesions. Stress fibers are bundles of about 20

filaments linked via non-muscle myosin II. ''In vitro'' studies showed that mDia1 assembled

stress fiber Stress fibers are contractile actin bundles found in non-muscle cells. They are composed of actin (microfilaments) and non-muscle myosin II (NMMII), and also contain various crosslinking proteins, such as α-actinin, to form a highly regulated ...

s downstream of

. Live-cell imaging analysis revealed that mDia1 indeed assembles stress fibers, which are connected at one of their ends to

focal adhesion In cell biology, focal adhesions (also cell–matrix adhesions or FAs) are large macromolecular assemblies through which mechanical force and regulatory signals are transmitted between the extracellular matrix (ECM) and an interacting cell. More ...

s, where actin polymerization occurs. Stress fiber assembly at focal adhesions by mDia1 was later shown to promote their growth and stabilization, suggesting mDia1 exerts effects on the interactions of cells with their environment.

Formins Formins (formin homology proteins) are a group of proteins that are involved in the polymerization of actin and associate with the fast-growing end (barbed end) of actin filaments. Most formins are Rho-GTPase effector proteins. Formins reg ...

regulate endocytosis. mDia 1 localizes to

endosomes Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membrane ca ...

and regulates phagocytic cup formation in

macrophages Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...

. mDia1 (and mDia2) seems to stabilize microtubules by decreasing the

tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...

subunit exchange at their plus ends. The exact mechanism is not fully understood yet. However, the affinity of formins for

are much higher than for microtubules. By catalyzing actin

and stabilizing microtubules, mDia1 plays also an important role for cell migration.

Discovery

mDia1 was discovered as p140mDia1 by Watanabe ''et al.'' in 1997 as a downstream effector of

. A mouse embryo cDNA library was screened to identify a RhoA-GTP-binding protein using a

yeast two-hybrid Two-hybrid screening (originally known as yeast two-hybrid system or Y2H) is a molecular biology technique used to discover protein–protein interactions (PPIs) and protein–DNA interactions by testing for physical interactions (such as bind ...

system. Further it was shown that p140mDia1 binds to the GTP-bound form of RhoA only by precipitation from Swiss 3T3 cell lysates. Watanabe ''et al.'' could also show the interaction of p140mDia1 with profilin and the colocalization of RhoA, p140mDia and profilin in membrane ruffles of motile cells. A subsequent study in 1997 by Bione ''et al.'' established a link between human DIA and oogenesis, with a defect in the gene leading to premature ovarian failure.

References

External links

*{{Commonscatinline
Protein Database
Cell cycle Genes mutated in mice Mouse proteins