Essentially, the main function of the YadA domain is to help cell adhesion and to increase virulence. YadA is a collagen-binding outer membrane protein. It forms the fibrillar matrix on the bacterial cell surface. This aids cell attachment and helps the bacteria invade eukaryotic cells. Additionally, by forming the fibrillar matrix, the YadA domain protects the bacteria by facilitating agglutination resistance, serum resistance, complement inactivation and phagocytosis resistance. The importance of adhesins to YadA function and Yersinia survival is huge. Attachment further allows more interactions and increase of biofilm formation to aid bacterial colonization. In Yersinia, it helps initiate the infectious process in host cells and are critical virulence factors. Additionally, bacteria have the ability to regulate adhesin expression, meaning that when Yersinia no longer requires YadA, it can be turned off. Furthermore, YadA expression is mainly temperature regulated, at 37 degrees Celsius. It also has two molecular regulators: an activator, VirF and a repressor, YmoA.
Substrate adhesion
The YadA protein domain adheres to the following substrates:
The C-terminal domain consists of 120 amino acids which belong to a family of surface-exposed bacterial proteins. The YadA C-terminal domain has a particular function in translocating the trimeric N-terminal passenger domain to the exterior of the membrane and is also responsible for trimerisation.
YadA is a homotrimeric outer membrane protein which forms part of the fibrillar matrix. Simplistically, this means the protein is made of three of the same subunits, on the outer surface of the membrane. The surface is entirely covered in the YadA lollipop structures. made of a short C-terminal membrane anchor, an 18 nm long coiled-coil stem and a 5 nm long N-terminal globular head structure consisting of a left-handed parallel beta roll. YadA is an example of an oligomeric coiled-coil adhesin. The Oca protein families are a subset of autotransporters, also known as the type Vc or trimeric autotransporters. Trimerization is thought to involve the coiled-coil stem and the C-terminal membrane anchor, which forms a 12-strand beta-barrelfrom the four transmembranebeta-strands of the three monomers. This beta-barrel would form a pore-like structure through which the N-terminal head and coiled helical domains of the three monomer chains exit to the cell surface. The YadA protein domain, is a form of trimeric autotransporter adhesins. Each TAA must consist of a head, stalk and a beta-barrel membrane anchor.
History
YadA, an adhesin from Yersinia, was the first member of this family to be characterised. UspA2 from Moraxella was second. The Eib immunoglobulin-binding proteins from Escherichia coli were third, followed by the DsrA proteins of Haemophilus ducreyi, amongst others.