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. 3-methyl-(2E)-butenoyl-CoA + ATP + hydrogencarbonate = 3-methyl-(2E)-glutaconyl-CoA + ADP + H(+) + phosphate Binds 2 manganese ions per subunit. Amino-acid degradation; L-leucine degradation; (S)-3-hydroxy-3-methylglutaryl-CoA from 3-isovaleryl-CoA: step 2/3. Probably a heterodimer composed of biotin-containing alpha subunits and beta subunits. Induced during senescence.