Biological complexity: exquisite attack needle design (Introduction)

by David Turell , Wednesday, May 12, 2021, 23:21 (1291 days ago) @ David Turell

An elegant research paper picking apart a very complex protein attack needle:

https://www.nature.com/articles/s41467-021-21143-1

"Abstract
Many bacterial pathogens rely on virulent type III secretion systems (T3SSs) or injectisomes to translocate effector proteins in order to establish infection. The central component of the injectisome is the needle complex which assembles a continuous conduit crossing the bacterial envelope and the host cell membrane to mediate effector protein translocation. However, the molecular principles underlying type III secretion remain elusive. Here, we report a structure of an active Salmonella enterica serovar Typhimurium needle complex engaged with the effector protein SptP in two functional states, revealing the complete 800Å-long secretion conduit and unraveling the critical role of the export apparatus (EA) subcomplex in type III secretion. Unfolded substrates enter the EA through a hydrophilic constriction formed by SpaQ proteins, which enables side chain-independent substrate transport. Above, a methionine gasket formed by SpaP proteins functions as a gate that dilates to accommodate substrates while preventing leaky pore formation. Following gate penetration, a moveable SpaR loop first folds up to then support substrate transport. Together, these findings establish the molecular basis for substrate translocation through T3SSs and improve our understanding of bacterial pathogenicity and motility.

Introduction
Many important human pathogens including Salmonella, Shigella, Yersinia, and enteropathogenic Escherichia coli employ a conserved, virulent type III secretion system (T3SS), also commonly referred to as the injectisome, to deliver a pleiotropic arsenal of proteins into target eukaryotic cells. These proteins modulate host cell signal transduction processes to establish a biological niche within the host, making T3SSs crucial virulence determinants. Yet, the precise mechanisms that allow these secretion systems to facilitate unfolded protein transport across the bacterial envelope and into the host cell while maintaining bacterial membrane remain integrity poorly understood. Therefore, visualizing the translocation process at the molecular level is essential for our understanding of host–pathogen biology and the development of novel therapies targeting bacterial infection.

"The injectisome is a large molecular machine, over 3.6 MDa in mass, spanning across the inner and outer bacterial membranes with an extracellular filamentous appendage extending out to target host cells. Chaperones present effector proteins in a non-globular, secretion-competent state to a cytoplasmic sorting platform complex, which sorts and loads effectors into the export apparatus (EA) subcomplex located inside the membrane-bound basal body. Extending from the EA is a long, helical needle filament, capped by a tip complex that contacts the host cell membrane via assembly of a translocon pore. The basal body and the needle filament, collectively termed the needle complex, function as a continuous conduit for effector protein translocation from the prokaryotic to the host cell cytoplasm.

***

"In this work we report cryo-EM structures of an active Salmonella enterica sv. Typhimurium needle complex engaged with a SptP3x-GFP substrate, revealing the complete 800 Å-long secretion conduit and unraveling the critical role of the EA in substrate transport. Unfolded substrates enter the EA through a hydrophilic constriction formed by SpaQ proteins, which enables side chain-independent transport, providing a rationale for the heterogeneity and structural disorder of signal sequences in T3SS effector proteins. Above, a methionine gasket formed by five SpaP proteins functions as a gate that dilates to accommodate substrates but prevents leaky pore formation to maintain the physical boundaries of compartments separated by a biological membrane. Above the gate, a moveable SpaR loop first folds up to then support substrate transport through the needle complex channel. Together, these findings establish the molecular basis for substrate translocation through T3SSs, improving our understanding of bacterial pathogenicity and motility of flagellated bacteria, and pave the way for the development of novel concepts combating bacterial infections."

Comment: Read these paragraphs slowly as your eyes roll back. Extreme complexity shown beautifully in the diagrams which should be seen to appreciate this DID NOT happen by natural chance events.

"Researchers in Germany recently examined the T3SS in more detail, finding more complexities of engineering that should arouse the observer’s awe. For instance, these machines can fire effector proteins at the rate of 7 to 60 molecules per second! The machine resembles a dart gun in the bacterial cell wall that is loaded from the cytoplasm and can tunnel into a neighboring cell, probably with the aid of a pioneer translocator that opens a hole in the host membrane."

https://evolutionnews.org/2021/05/closer-look-at-the-t3ss-reveals-design/

It is a wild west shoot 'em out at teh bacterial level.