Actin and its interacting proteins form an indispensable network in cells; a dynamic filament system that is crucial for locomotion, cell division, shape, and various forms of intracellular motility. Control of the site-specific assembly and disassembly of actin filaments is critical for directing these activities to the appropriate position in the cell. Microrganisms can trigger rearrangements of the actin cytoskeleton, typified by enterics like Listeria monocytogenes. Upon entering a living cell, the organisms use the actin network to locomote in the host cytoplasm or to enter and infect neighboring cells. Other microorgansims such as Shigella, Rickettsia and vaccinia viruses also share Listeria's modus operandi.
The gram negative enteric Escherichia coli also remodels the actin cytoskeleton. Enteropathogenic E. Coli (EPEC) and Enterohemorrhagic E. coli (EHEC) attach to intestinal cell surfaces and induce loss or effacement of the microvilli, resulting in diarrhea. Colonization of intestinal cells by EPEC and EHEC is characterized by: 1) extracellular surface attachment; 2) microvilli effacement; 3) microorganism adhesion and 4) pedestal formation. Numerous actin binding proteins, alpha-actinin, Arp2/3, ezrin, talin, villin and WASP concentrate beneath the EPEC attachments. The organism rests atop the pedestal of actin filaments and glides over the host cell surface while attached to the extracellular membrane. We know that actin polymerization is required for the movement, but we do not understand the actin dynamics and other proteins involved to produce movement.
As part of the infection process, the EPEC encoded protein Tir (Trans intimin receptor) is secreted and injected into the host eukaryotic cell membrane via the Type III secretion system. Tir is a protein with two predicted transmembrane spanning regions resembling a hairpin structure. Structurally, the protein can be divided into three distinct domains: a predicted alph-helix N-terminal domain of 233 amino acids; a second centralized domain of 106 amino acids (includes one tyrosine) and a third predicted alpha-helix domain of 165 amino acids (contains three tyrosines). The membrane positions of these three protein domains are well established with the first and third domains exposed to the cytoplasm, and the second domain extracellular to the infected cell.
Amino acid sequence analysis of Tir shows little homology with other known proteins. We are specifically interested in the molecular interactions between Tir and host cell proteins. The continued understanding of the molecular mechanisms involved in host cell/pathogen interactions is crucial to our diagnosis and treatment of infectious disease.