In addition to the intracellular transport of particles (cargo) along microtubules, there are in the cell two actin-based transport systems. In the actomyosin system the transport is driven by myosin, which moves the cargo along actin microfilaments. This transport requires the hydrolysis of ATP in the myosin molecule motor domain that induces conformational changes in the molecule resulting in the myosin movement along the actin filament. The other actin-based transport system of the cell does not involve myosin or other motor proteins. This system is based on a unidirectional actin polymerization, which depends on ATP hydrolysis in actin polymers and is initiated by proteins bound to the surface of transported particles. Obligatory components of the actin-based transport are proteins of the WASP/Scar family and a complex of Arp2/3 proteins. Moreover, the actin-based systems often contain dynamin and cortactin. It is known that a system of actin filaments formed on the surface of particles, the so-called “comet-like tail”, is responsible for intracellular movements of pathogenic bacteria, micropinocytotic vesicles, clathrin-coated vesicles, and phagosomes. This movement is reproduced in a cell-free system containing extract of Xenopus oocytes. The formation of a comet-like structure capable of transporting vesicles from the plasma membrane into the cell depth has been studied in detail by high performance electron microscopy combined with electron tomography. A similar mechanism provides the movement of vesicles containing membrane rafts enriched with sphingolipids and cholesterol, changes in position of the nuclear spindle at meiosis, and other processes. This review will consider current ideas about actin polymerization and its regulation by actin-binding proteins and show how these mechanisms are realized in the intracellular actin-based vesicular transport system.

The article reviews the discovery, properties and functional activities of new bacterial enzymes, proteases grimelysin (ECP 32) of Serratia grimesii and protealysin of Serratia proteamaculans, characterized by both a highly specific “actinase” activity and their ability to stimulate bacterial invasion. Grimelysin cleaves the only polypeptide bond Gly42-Val43 in actin. This bond is not cleaved by any other proteases and leads to a reversible loss of actin polymerization. Similar properties were characteristic for another bacterial protease, protealysin. These properties made grimelysin and protealysin a unique tool to study the functional properties of actin. Furthermore, bacteria Serratia grimesii and Serratia proteamaculans, producing grimelysin and protealysin, invade eukaryotic cells, and the recombinant Escherichia coli expressing the grimelysin or protealysins gene become invasive. Participation of the cellular c-Src and RhoA/ROCK signaling pathways in the invasion of eukaryotic cells by S. grimesii was shown, and involvement of E-cadherin in the invasion has been suggested. Moreover, membrane vesicles produced by S. grimesii were found to contain grimelysin, penetrate into eukaryotic cells and increase the invasion of bacteria into eukaryotic cells. These data indicate that the protease is a virulence factor, and actin can be a target for the protease upon its translocation into the host cell.

Protealysin is a thermolysin‐like protease of Serratia proteamaculans capable of specifically cleaving actin, which correlates with the invasive activity of these bacteria. Here, we show that inactivation of the protealysin gene does not inhibit invasion but, in contrast, leads to a twofold increase in the S. proteamaculans invasive activity. By mass spectrometry, we identified the outer membrane protein OmpX as a substrate of protealysin. Recombinant E. coli carrying the OmpX gene truncated by 40 N‐terminal residues or both the OmpX and protealysin genes, in contrast to the full‐length OmpX, do not increase adhesion of these bacteria, indicating that the 40 N‐terminal residues of OmpX are indispensable for S. proteamaculans invasion. Our results show that both protealysin and its substrates can stimulate Serratia invasion.

Serratia proteamaculans synthesizes the intracellular metalloprotease protealysin. This work was aimed at searching for bacterial substrates of protealysin among the proteins responsible for replication and cell division. We have shown that protealysin unlimitedly cleaves the SOS response protein RecA. Even 20% of the cleaved RecA in solution appears to be incorporated into the polymer of uncleaved monomers, preventing further polymerization and inhibiting RecA ATPase activity. Transformation of Escherichia coli with a plasmid carrying the protealysin gene reduces the bacterial UV survival up to 10 times. In addition, the protealysin substrate is the FtsZ division protein, found in both E. coli and Acholeplasma laidlawii, which is only 51% identical to E. coli FtsZ. Protealysin cleaves FtsZ at the linker between the globular filament-forming domain and the C-terminal peptide that binds proteins on the bacterial membrane. Thus, cleavage of the C-terminal segment by protealysin can lead to the disruption of FtsZ’s attachment to the membrane, and thereby inhibit bacterial division. Since the protealysin operon encodes not only the protease, but also its inhibitor, which is typical for the system of interbacterial competition, we assume that in the case of penetration of protealysin into neighboring bacteria that do not synthesize a protealysin inhibitor, cleavage of FtsZ and RecA by protealysin may give S. proteamaculans an advantage in interbacterial competition.