This protein is a multifunctional enzyme, able to activate and polymerize the amino acids Pro, Val, Orn and Leu. Activation sites for these AA consist of individual domains. Binds 4 phosphopantetheines covalently. Antibiotic biosynthesis; gramicidin S biosynthesis. Large multienzyme complex of GrsA and GrsB. Consists of four modules, and harbors a putative thioesterase domain at its C-terminal end. Each module incorporates one amino acid into the peptide product and can be further subdivided into domains responsible for substrate adenylation, thiolation, condensation (not for the initiation module), and epimerization (optional), and N methylation (optional). Belongs to the ATP-dependent AMP-binding enzyme family.