Autolysins are endogenous lytic

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

s that break down the

peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid Cell wall#Bacterial_cell_walls, cell wall (murein sac ...

components of

biological cells The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a membrane, and contains many biomolecules such as proteins, DNA and RNA, as well as many small molecules of nutrients ...

which enables the separation of daughter cells following cell division. They are involved in cell growth, cell wall metabolism, cell division and separation, as well as peptidoglycan turnover and have similar functions to lysozymes. Autolysin is formed from the precursor gene, Atl. Amidases (EC 3.5.1.28), gametolysin (EC 3.4.24.38), and glucosaminidase are considered as types of autolysins.

Function and mechanisms

Autolysins exist in all

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

containing

and are potentially considered as lethal enzymes when uncontrolled. They target the glycosidic bonds as well as the cross-linked peptides of the peptidoglycan matrix. The peptidoglycan matrix functions for cell wall stability to protect from turgor changes and carries out function for immunological defense. These enzymes break down the peptidoglycan matrix in small sections to allow for peptidoglycan biosynthesis. Autolysins breaks down old peptidoglycan which allows for the formation of newer peptidoglycan for cell growth and elongation. This is called cell wall turnover. Autolysins do this by hydrolyzing the β-(1,4) glycosidic bond of the peptidoglycan cell wall and the linkage between N-acetylmuramoyl residues and L-amino acid residues of certain cell-wall glycopeptides. This enzyme catalyses the following

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...

: : Cleavage of the proline- and hydroxyproline-rich proteins of the ''

Chlamydomonas ''Chlamydomonas'' is a genus of green algae consisting of about 150 speciesSmith, G.M. 1955 ''Cryptogamic Botany Volume 1. Algae and Fungi'' McGraw-Hill Book Company Inc of unicellular flagellates, found in stagnant water and on damp soil, ...

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...

; also cleaves azocasein,

gelatin Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...

and Leu-Trp-Met-Arg-Phe-Ala This glycoprotein is present in '' Chlamydomonas reinhardtii''

gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

Gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bac ...

bacteria regulate autolysins with teichoic acid molecules attached to the tetrapeptide of the peptidoglycan matrix. The

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...

s complestatin and corbomycin prevent autolysin from remodeling the cell wall by binding to peptidoglycan, therefore stopping bacterial growth. The amide linkages between stem peptide and lactyl moiety of muramoyl residue are cleaved by N-acetylmuramoyl-l-alanine amidases and partakes in cell separation and the dissociation of the cell septum. There are 5 types of autolysins that contribute to cell separation of daughter cells, LytC, LytD, LytE, and LytF. In a study conducted with mice, those that were immunized with autolysin were able to survive longer than the infected mice. This study was able to support as evidence autolysin's contribution in virulence and potential for vaccine antigen.

Lysis of mother cell

LytC and CwlC are two amidases from the LytC family that hydrolyze the peptidoglycan of the mother cell wall to allow for the release of the mature endospore. CwlC is directly found in the mother cell wall.

Motility

Expression of lytC, lytD, and lytF genes together leads to flagellar motility and is controlled by the activity of the chemotaxis sigma factor, σ^D. The activity of this sigma factor peaks at the start of the stationary phase.

Potential lethality

Autolysins are naturally produced by peptidoglycan containing bacteria, but excessive amounts will degrade the peptidoglycan matrix and cause the cell to burst due to osmotic pressure. Previous studies have found that the byproducts of autolysin during cell wall breakdown are highly immunogenic. When observed in the bacteria, ''

Bacillus subtilis ''Bacillus subtilis'', known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus '' Baci ...

'', there were potentially lethal amounts of autolysin found in the cell walls. In ''Streptococcus pneumoniae'' it was found that N-acetylmuramoyl-l-alanine amidase, a cell wall autolysin, could assist in pathogenesis due to its ability to break down the wall or lyse a portion of the invading pneumococci and release potentially lethal toxins into the cell. Researchers studied the function, structure, and cloning ability through ''Escherichia coli'' and also determined its nucleotide sequence.

Families

LytC amidase family

LytC

LytC as well as LytD are considered as two major autolysins that contribute to vegetative cell wall growth and account for 95% of the autolytic activity in B. subtilis. LytC is found in the cell wall. LytB, a non-autolysin, was found to enhance LytC activity. LytC and LytA interact and function together for lysis and cell death.

CwlC

CwlC is found in the mother cell wall and functions for the lysis of the mother cell wall. CwlC does not have a signal sequence but participates in late sporulation and is present in the cell wall. It was found in B. subtilis that CwlC is able to hydrolyze both vegetative cell walls and spore peptidoglycan.

LytD glucosaminidase family

This family of autolysin consist of only LytD itself. LytD functions for vegetative growth. Autolytic activity is found within the C-terminal region with catalytic domain homologous to the glucosaminidase domain. LytD is found in the cell wall. LytD activity was studied in B. subtilis and glucosaminidase activity was found in mature glycan strands due to the presence of MurNAc at the nonreducing ends.

References

External links

* {{Portal bar, Biology, border=no EC 3.4.24 Bacterial enzymes