Component of the high-affinity leucine, isoleucine, valine transport system I (LIV-I), which is operative without Na(+) and is specific for alanine and threonine, in addition to branched-chain amino acids (PubMed:2509433). Binds L-leucine, L-isoleucine, L-valine, L-threonine and L-alanine with nanomolar affinities. Can also bind L-homoserine with high affinity (PubMed:31627455). Belongs to the leucine-binding protein family.