When you see schematics of flagella and pili in research papers or text books, it's easy to think that we know the structure, and that we've probably known it for a long time. Structural biologists will tell you that this often isn't the case and even when structures are solved and published, there can still be ambiguity about how the proteins look or behave in native conditions. 

A recent paper published by a team in India discovered a compelling mechanism for how SpaE pillin subunits are incoporated into the pilus. The reason that this is important is that SpaE is a component of the Sortase-dependent pilli which are relatively very simple when compared with chaperone-usher, type IV or type V pili. Not understanding the structure and mechanism of one of the simplest gram-postivie pili highlights the lack of information we have at the atomic level. However, the research team led by Vengadesan Krishnan produced an extremely high resolution crystal structure at the ESRF synchrotron which allowed them to propose a potential mechanism for the extension of the sortase-dependent pilus. 

Why is this important? Well the more high quality protein structures we have, the better chance we have of generating new high quality structures in the future. These particular pili are also present in a number of gut bacteria, from pathogens to commensals. Understanding the subtle molecular interactions can help us add to the knowledge on how complex microbial communities impact our health.

Read the full text of the article here

Abstract

For some Gram-positive genera and species, the long-extended and adhesive sortase-dependent pilus plays an essential role during host colonization, biofilm formation, and immune modulation. Lactobacillus rhamnosus GG is a gut-adapted commensal strain that harbors the operonic genes for the SpaCBA and SpaFED pili, both being comprised of three different protein subunits termed the backbone, tip, and basal pilins. Crystal structures of the backbone pilins (SpaA and SpaD) have recently been solved, and here we describe the high-resolution (1.5 Å) structural determination of the SpaE basal pilin. SpaE consists of two immunoglobulin-like CnaB domains, with each displaying a spontaneously formed internal isopeptide bond, though apparently slow forming in the N-terminal domain. Remarkably, SpaE contains an atypically lengthy unstructured C-terminal tail, along with an YPKN pilin motif peptide, which is normally reserved for backbone subunits. Based on our analysis of the crystal structure data, we provide a molecular model for the basal positioning of the SpaE pilin within the SpaFED pilus.

Reference

J Struct Biol. 2019 Apr 23. pii: S1047-8477(19)30082-6. doi: 10.1016/j.jsb.2019.04.016. [Epub ahead of print]
Crystal structure of basal pilin SpaE reveals the molecular basis of its incorporation in the lactobacillar SpaFED pilus.
Megta AK, Mishra AK, Palva A, von Ossowski I, Krishnan V.