Biotin-attachment subunit of the 3-methylcrotonyl-CoA carboxylase, an enzyme that catalyzes the conversion of 3- methylcrotonyl-CoA to 3-methylglutaconyl-CoA, a critical step for leucine and isovaleric acid catabolism. {ECO:0000269|PubMed:17360195, ECO:0000269|PubMed:32561715}. This is the function of MCCC1 (methylcrotonyl-CoA carboxylase subunit 1, ENSG00000078070).