Involved in the maturation of [NiFe] hydrogenases. Required for nickel insertion into the metal center of the hydrogenase. Exhibits a low intrinsic GTPase activity, which is essential for nickel insertion (By similarity). Is able to bind 4 nickel ions per subunit. Can also bind zinc (PubMed:7928968). Is synthesized in microaerobic vegetative cells and pea bacteroids but not in aerobic vegetative cells. A histidine-rich region at the N-terminus is proposed to play a role in nickel binding, both in solution and in chelated form. Insertion mutant lacks any hydrogenase activity in symbiosis with peas, but is still able to synthesize the polypeptide for the hydrogenase large subunit. Belongs to the SIMIBI class G3E GTPase family. HypB/HupM subfamily.