Research Notes: Rv3660c (Ssd) - Mycobacterium tuberculosis H37Rv

Gene Identity

Rv3660c encodes a septum site determining protein (Ssd) in M. tuberculosis. It is a member of the ParA/MinD/Ssd WACA (Walker A Cytoskeletal ATPase) family but is distinct from the true E. coli MinD homologue in M. tuberculosis (Rv1708).

UniProt: P9WKX7. 350 amino acids. COG0455 (ATPases involved in chromosome partitioning). Contains a deviant Walker A motif (P-loop NTPase fold).

Key Findings from Literature

Septum formation inhibitor (PMID:21504606, England et al. 2011)

This is the primary characterization study. Rv3660c was identified as encoding a septum site determining protein (Ssd) by consensus-modeling bioinformatics.

• Overexpression of Rv3660c in both M. smegmatis and M. tuberculosis inhibited septum formation, resulting in elongated filamentous cells devoid of septa PMID:21504606
• Disruption by transposon insertion gave a minicell morphology, confirming its inhibitory role PMID:21504606
• Overexpression induced the dormancy (Dos) regulon and alternative sigma factors (SigF, SigG, SigH, SigI, SigL, SigM) PMID:21504606
• This links septum regulation to the adaptive metabolic response associated with non-replicating persistence and virulence PMID:21504606

Distinction from Rv1708 as MinD homologue (PMID:37526955, Kishore et al. 2023)

Kishore et al. showed that Rv1708 (not Rv3660c) is the true E. coli MinD homologue in M. tuberculosis:

• Rv1708/MSMEG_3743 fully complemented the E. coli MinD mutant (minicell phenotype), while Rv3660c showed only partial complementation PMID:37526955
• Structural analysis showed Rv1708 overlaps better with E. coli MinD than Rv3660c does PMID:37526955
• Rv3660c has a unique N-terminal domain (CheY-like, IPR059050) absent in E. coli MinD PMID:37526955
• MSMEG_3743 (Rv1708 homologue) showed ATPase activity consistent with Walker A motif PMID:37526955

Essential for growth (PMID:12657046, Sassetti et al. 2003)

Rv3660c was identified as essential for optimal growth by transposon site hybridization (TraSH) PMID:12657046

Induction by FtsZ inhibitors (PMID:16735741, Slayden et al. 2006)

Rv3660c was induced by albendazole and thiabendazole (GTPase inhibitors of FtsZ), linking it to septum formation regulation PMID:16735741

Drug target (PMID:19099550, Raman et al. 2008)

Identified as a high-confidence drug target through interactome, reactome, and genome-scale structural analysis pipeline (targetTB).

Domain Architecture

• N-terminal CheY-like domain (IPR059050, residues 1-109): protein interaction module
• Rv3660c family domain (IPR022521, residues 2-338): defines this specialized lineage
• ParA/MinD ATPase domain (IPR050625, residues 100-334): P-loop NTPase with Walker A/B motifs
• P-loop NTPase superfamily (IPR027417): structural fold for nucleotide hydrolysis

PANTHER Classification

PANTHER family PTHR43384 classifies Rv3660c as related to "septum site-determining protein MinD homolog" (SF11). The IBA annotations are transferred from this family via PANTHER:PTN000344128, with E. coli MinD (P0AEZ3) and a plant homologue (G3XD64) as reference proteins.

Functional Summary

Rv3660c/Ssd is a septum site determining protein that inhibits cell division septum formation in M. tuberculosis. It belongs to the ParA/MinD ATPase superfamily but is functionally distinct from the true MinD homologue (Rv1708). Its key biological roles are:

1. Negative regulation of division septum assembly via inhibition of FtsZ polymerization
2. Link between cell cycle checkpoint regulation and dormancy/persistence responses
3. Essential for growth (likely for proper cell division regulation)

The protein acts in the cytosol and contains ATP binding/hydrolysis capacity via a conserved Walker A motif, though direct ATPase activity has only been demonstrated for the M. smegmatis Rv1708 homologue (MSMEG_3743), not for Rv3660c itself.