TCDB is operated by the Saier Lab Bioinformatics Group

9.B.159 The Inclusion Membrane Protein (IncA) Family

The IncA family includes several proteins that are secreted via Type III Secretion Systems (TC# 3.A.6) in chlamydial species.  No member of this family has been identified with certainty outside of the Chlamydiae.  The proteins enterred into TCDB as IncA proteins usually have a 2 or 4 TMS topology but are in general not demonstrably homologous using TC BLAST.  They are very divergent in sequence, but they all belong to the IncA family of Pfam.  They may be related to mitochodrial Ca2+ uniporters and prokaryotic Mg2+ channels (TC# 1.A.77), and consequently, these IncA homologues may function as divalent cation channels (Lee et al. 2014). The N-terminal regions appear to be more similar than the remaining parts of the protein.  9.B.159.2.3 retrieves members of 7 sub-families including its own.

Inc proteins may assemble in the membrane of the inclusion (which separates the bacteria from the host cell cytoplasm) to form specific multi-molecular complexes (Gauliard et al. 2015).  IncA plays a critical role in both protecting the inclusion from lysosomal fusion and inducing the homotypic fusion of inclusions. Within IncA are two regions homologous to eukaryotic SNARE (soluble N-ethylmaleimide-sensitive factor attachment receptor) domains referred to as SNARE-like domain 1 (SLD1) and SNARE-like domain 2 (SLD2). The functional core of IncA retains the ability to both inhibit SNARE-mediated fusion and promote the homotypic fusion of Chlamydia inclusions.This core region of 2 TMS proteins is composed almost entirely of α-helices and assembles into stable homodimers in solution (Ronzone et al. 2014).

Many intracellular bacteria, including Chlamydia, establish a parasitic membrane-bound organelle inside the host cell that is essential for the bacteria's survival. Chlamydia trachomatis forms inclusions that are decorated with poorly characterized membrane proteins known as Incs. The prototypical Inc, called IncA, enhances Chlamydia pathogenicity by promoting the homotypic fusion of inclusions and shares structural and functional similarity to eukaryotic SNAREs. Cingolani et al. 2019 presented the atomic structure of the cytoplasmic domain of IncA, which reveals a non-canonical four-helix bundle. Structure-based mutagenesis, molecular dynamics simulation, and functional cellular assays identified an intramolecular clamp that is essential for IncA-mediated homotypic membrane fusion during infection. Solithromycin (Sol) impedes homotypic fusion of Chlamydia-containing vacuoles and reduces secretion of the type III secretion (T3S) effector IncA (Gao et al. 2018).

VAMP3 and VAMP4 are eukaryotic SNARE proteins that mediate membrane fusion and are recruited to the inclusion to facilitate inclusion expansion (Bui et al. 2020). VAMPs 3 and 4 are probably recruited by Incs. In chlamydia-infected cells, five Incs temporally and transiently interact with VAMP3. Loss of incA or ct813 expression altered VAMP3 localization to the inclusion. Thus, these transient interactions likely contribute to the chlamydial developmental cycle (Bui et al. 2020).

References associated with 9.B.159 family:

Bui, D.C., L.M. Jorgenson, S.P. Ouellette, and E.A. Rucks. (2020). EUKARYOTIC SNARE VAMP3 DYNAMICALLY INTERACTS WITH MULTIPLE CHLAMYDIAL INCLUSION MEMBRANE PROTEINS. Infect. Immun. [Epub: Ahead of Print] 33229367
Cingolani, G., M. McCauley, A. Lobley, A.J. Bryer, J. Wesolowski, D.L. Greco, R.K. Lokareddy, E. Ronzone, J.R. Perilla, and F. Paumet. (2019). Structural basis for the homotypic fusion of chlamydial inclusions by the SNARE-like protein IncA. Nat Commun 10: 2747. 31227715
Gao, L., Y. Wang, Z. Hua, E. Liu, and L. Shen. (2018). Potency of Solithromycin against Fast- and Slow-Growing Chlamydial Organisms. Antimicrob. Agents Chemother. 62:. 29891601
Gauliard, E., S.P. Ouellette, K.J. Rueden, and D. Ladant. (2015). Characterization of interactions between inclusion membrane proteins from Chlamydia trachomatis. Front Cell Infect Microbiol 5: 13. 25717440
Lee, A., A. Vastermark, and M.H. Saier, Jr. (2014). Establishing homology between mitochondrial calcium uniporters, prokaryotic magnesium channels and chlamydial IncA proteins. Microbiology 160: 1679-1689. 24869855
Ronzone, E., J. Wesolowski, L.D. Bauler, A. Bhardwaj, T. Hackstadt, and F. Paumet. (2014). An α-helical core encodes the dual functions of the chlamydial protein IncA. J. Biol. Chem. 289: 33469-33480. 25324548