mlaB (P64602, E. coli K-12) review notes

Deep research

2026-10-23: just deep-research ECOLI mlaB --provider perplexity was run once and
failed ("Provider 'perplexity' not available. Available: falcon, openscientist").
No deep-research file was generated; this review is based on the cached
publications listed below (full text unless noted) and the UniProt record.

MlaB (YrbB, b3191), 97 aa, a single STAS domain (PROSITE PS50801, Pfam PF13466);
last gene of the mlaFEDCB operon. Cytoplasmic; no TM helix or signal peptide.
Binds the side of each MlaF NBD: "MlaB, a STAS domain protein, binds the ABC
nucleotide binding domain, MlaF, and is required for its stability" [PMID:32602838,
full text]. The MlaB binding site is ~30 A from the ATP site; MlaB subunits do not
contact each other and have "presumed minimal contacts" with other transporter
subunits PMID:32602838.
MlaFEDB cryo-EM: "On the cytoplasmic side, each of the MlaF ATPase subunits is bound
to MlaB" PMID:33236984. MlaB does not line the lipid pathway, which runs through
MlaE and MlaD PMID:33236984.

Thong et al. 2016 [PMID:27529189, full text]: "MlaB is necessary for both the assembly
and activity of the ABC transporter, likely by modulating MlaF structure and
stability". Without MlaB, MlaFE could not be isolated. MlaB T52A does not affect
MlaFEB assembly but abolishes ATPase activity, and mlaB T52A does not complement
SDS-EDTA sensitivity.
Kolich et al. 2020 PMID:32602838: MlaF cannot be expressed/purified alone, but
MlaFB co-expression gives good yield; "These data suggest that MlaB stabilizes MlaF".
MlaF over-expression fully suppresses the mlaB knockout: "MlaB may have no essential
function in Mla lipid transport apart from its role in promoting MlaF stability
and/or association with the MlaFEDB complex". Thr52 lies at the MlaF-MlaB interface
and is the conserved Ser/Thr phosphorylation site of STAS domains; phosphoregulation
is hypothesized, not shown.
Interpretation: MlaB is a stoichiometric, non-catalytic accessory subunit. Its
documented activity is binding and stabilizing MlaF (GO:0050821 protein
stabilization, process), enabling MlaFB to assemble with MlaED. It does not contact
phospholipid, so "enables phospholipid transfer activity" overstates it; at most it
contributes to the complex activity. GO lacks a specific MF term for "ABC ATPase
subunit stabilizer"; no MF is asserted with enables in core_functions.

Tn5 screen for hyperlethality to nalidixic acid; yrbB::Tn5 was among 14 hits, also
more readily killed by mitomycin C, UV and H2O2; complementation was observed for
tested mutants including yrbB. The authors note the yrbB mutation does not change
bacteriostatic susceptibility or permeability to most compounds. These are indirect,
envelope-related phenotypes; nothing places MlaB in a DNA-damage response pathway.
PAINT PTN002185077 (PTHR35849, taxon Gammaproteobacteria) propagates
GO:0006974 DNA damage response, GO:0120014, GO:0120010 and GO:1990531, all seeded
solely by P64602 itself (interpro/panther/PTHR35849/PTHR35849-paint.tsv).

UniProt: "Cytoplasm" (ECO:0000305 PMID:19383799). EcoCyc IDA cytosol from proteomics
PMID:18304323 (full text; MlaB not found in cached text, presumably in supplementary
tables) and PMID:17309111 (abstract only). Functional location is the cytoplasmic face
of the IM, bound to MlaF in MlaFEDB.

PMID:29358647 (TatA-type Tat translocase paper) is the ComplexPortal NAS reference for
"membrane"; full text has no mention of Mla genes - miscited.
GO:1990531 is defined as a P-type ATPase (P4 flippase) complex; MlaFEDB is an ABC
transporter: GO:0043190 is the correct complex term.