Involved in the maturation of [NiFe] hydrogenases. Required for nickel insertion into the metal center of the hydrogenase (PubMed:12081959, PubMed:15995183). Mediates transfer of nickel, but not zinc, from the low-affinity metal-binding site in the GTPase domain of HypB to HypA (PubMed:23899293, PubMed:27951644). HypA is involved in maturation of hydrogenase 3. It may partially substitute for the function of HybF and vice versa (PubMed:12081959). May act as a scaffold for assembly of the nickel insertion proteins with the hydrogenase precursor protein after delivery of the iron center (PubMed:22016389). GDP-loaded state of HypB enhances HypA-HypB complex formation and nickel transfer. Homodimer (PubMed:15995183). Forms complexes with HypB and SlyD, and interacts with HycE, the large subunit of hydrogenase 3 (PubMed:15995183, PubMed:22016389, PubMed:23899293). Disruption of the gene prevents development of hydrogenase 3 activity, does not influence the level of hydrogenase 1 and leads to a considerable increase in hydrogenase 2 activity (PubMed:1482271). HypA-hybF double mutant is completely blocked in maturation of hydrogenases 1, 2 and 3. However, the inclusion of high nickel concentrations in the medium can restore limited activity of all three hydrogenases (PubMed:12081959). Belongs to the HypA/HybF family.