Gene Description

MccB is the carboxyltransferase beta subunit of bacterial 3-methylcrotonyl-CoA carboxylase. It accepts a carboxyl group from MccA-bound carboxybiotin and transfers it to 3-methylcrotonyl-CoA, forming 3-methylglutaconyl-CoA in distal L-leucine catabolism.

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0004485 methylcrotonoyl-CoA carboxylase activity	IEA GO_REF:0000120	ACCEPT	Summary: Correct molecular function of the assembled MCC complex. Reason: MccB is the substrate-binding carboxyltransferase subunit that contributes to this reaction.
GO:0006552 L-leucine catabolic process	IEA GO_REF:0000118	ACCEPT	Summary: Correct distal L-leucine catabolic pathway assignment. Reason: The MCC reaction follows Ivd in the conserved liu gene cluster.
GO:0016874 ligase activity	IEA GO_REF:0000002	KEEP AS NON CORE	Summary: Generic ligase activity is a broad parent of the specific carboxylase reaction. Reason: Retain this valid parent activity as non-core; GO:0004485 is the informative complex activity.
GO:1905202 methylcrotonoyl-CoA carboxylase complex	IEA GO_REF:0000118	ACCEPT	Summary: Correct complex membership. Reason: MccB is the beta subunit of methylcrotonyl-CoA carboxylase.

Core Functions

Carboxyltransferase beta subunit contributing to formation of 3-methylglutaconyl-CoA by the MCC complex.

Directly Involved In:

L-leucine catabolic process

In Complex:

methylcrotonoyl-CoA carboxylase complex

Supporting Evidence:

file:PSEPK/mccB/mccB-uniprot.txt
PANTHER; PTHR22855:SF13; METHYLCROTONOYL-COA CARBOXYLASE BETA CHAIN, MITOCHONDRIAL; 1.
PMID:16272386
The two subunits of MCase corresponded to liuB/liuD

References

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Suggested Experiments

Experiment: Reconstitute MccA-MccB MCC activity and test whether an mccB deletion blocks growth on leucine or isovalerate.

📚 Additional Documentation

Notes

(mccB-notes.md)

mccB curation notes

UniProt evidence: Q88FM4 carries the methylcrotonoyl-CoA carboxylase beta-chain PANTHER subfamily and carboxyltransferase domains. [file:PSEPK/mccB/mccB-uniprot.txt "PANTHER; PTHR22855:SF13; METHYLCROTONOYL-COA CARBOXYLASE BETA CHAIN, MITOCHONDRIAL; 1."].
The automated mitochondrial suffix belongs to the family label and is not a localization claim for this bacterial protein.
A characterized Pseudomonas methylcrotonoyl-CoA carboxylase consists of the two liuB/liuD products, supporting assignment of MccB as the beta carboxyltransferase partner of MccA. PMID:16272386.
MccB contributes to the complete MCC reaction and complex; it does not independently perform the MccA biotin-carboxylase/carrier half-reaction.

mccB