Catalyzes the transfer of endogenously produced octanoic acid from octanoyl-acyl-carrier-protein onto the lipoyl domain of GcvH, an intermediate carrier during protein lipoylation. Is also able to catalyze the reverse reaction. Octanoyl-CoA can also act as a substrate although very poorly. Does not display lipoate protein ligase activity, despite its sequence similarity to LplA. L-lysyl-[protein] + octanoyl-[ACP] = H(+) + holo-[ACP] + N(6)-octanoyl-L-lysyl-[protein] Protein modification; protein lipoylation via endogenous pathway; protein N(6)-(lipoyl)lysine from octanoyl-[acyl-carrier-protein]. Monomer. Cells lacking this gene are auxotrophic for lipoic acid when grown in minimal medium but grow as well as the wild-type strain in the presence of lipoic acid. The requirement for lipoic acid can be bypassed by addition of both acetate and branched-chain fatty acid (BCFA) precursors. The mutant cells are devoid of lipoylated proteins. In the reaction, the free carboxyl group of octanoic acid is attached via an amide linkage to the epsilon-amino group of a specific lysine residue of lipoyl domains of lipoate-dependent enzymes. The reaction proceeds via an octanoyl-thioester enzyme intermediate. Belongs to the octanoyltransferase LipM family.