The molybdenum storage protein - A bionanolab for creating experimentally alterable polyoxomolybdate clusters

摘要

Various N-2-fixing bacteria contain a cage -like molybdenum storage protein (MoSto) that deposits more than 100 Mo as discrete polyoxomolybdate (POMo) clusters. To explore the relationship between modifiable cage properties/preparation conditions on one side and the types of POMo clusters formed on the other we established a recombinant production system for MoSto of Azotobacter vinelandii and prepared site-specifically mutated, "in vivo-like and in vitro" POMo cluster-loaded and POMo cluster-free MoSto. Seven representative X-ray structures revealed highly different POMo clusters inside architecturally rather related MoSto cages. The only significant structural difference includes a small polypeptide segment, the beta-linker, which protrudes differently far into the cage interior. The beta-linker is positioned outwards in in vivo-like structures of MoSto (treated with ATP and Na2MoO4 during preparation) and inwards in in vitro structures (obtained after loading the purified POMo-cluster free MoSto with ATP and Na2MoO4). Non-covalent Mo8, Mo6-7 and Mo8-14 clusters are exclusively present in in vivo-like structures. Instead, in vitro structures contain a new well-defined Mo5-7 cluster II. The digit (s) behind Mo defines the (variable) number of metal atoms in the respective POMo clusters. In comparison to the native MoSto structures the L alpha 131H variant is characterized by a new non-covalent Mo3 cluster and a ca. 5 angstrom shifted Mo5-7 cluster II, by which the covalent Mo8 cluster becomes structurally modified. Altogether, the unique bionanolab in the MoSto cage is able to create a large variety of POMo clusters by expansions, fusions and positional/orientational variations of a few discrete polynuclear Mo-O building blocks.