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. In roots, cotyledons, leaves, flowers, ovaries, siliques and embryos. Temporal and spatial accumulation of the alpha and beta subunits during development at approximately equal molar ratios. May be due to exon skipping.