Identification with proteomics of novel proteins associated with the peribacteroid membrane of soybean root nodules

Soybean peribacteroid membrane (PBM) proteins were isolated from nitrogen-f ixing root nodules and subjected to N-terminal sequencing. Sequence data fr om 17 putative PBM proteins were obtained. Six of these proteins are homolo gous to proteins of known function. These include three chaperones (HSP60, BiP [HSP70], and PDI) and two proteases (a serine and a thiol protease), al l of which are involved in some aspect of protein processing in plants. The PBM homologs of these proteins may play roles in protein translocation, fo lding, maturation, or degradation in symbiosomes. Two proteins are homologo us to known, nodule-specific proteins from soybean, nodulin 53b and nodulin 26B, Although the function of these nodulins is unknown, nodulin 53b has i ndependently been shown to be associated with the PBM. All of the eight pro teins with identifiable homologs are likely to be peripheral rather than in tegral membrane proteins. Possible reasons for this apparent bias are discu ssed. The identification of homologs of HSP70 and HSP60 associated with the PBM is the first evidence that the molecular machinery for co- or post-tra nslational import of cytoplasmic proteins is present in symbiosomes, This h as important implications for the biogenesis of this unique, nitrogen-fixin g organelle.