Abstract
A regulatory protein from grass pea (Lathyrus sativus), LS-24, a close homolog of albumin 2 from garden pea (Pisum sativum) that is associated with polyamine biosynthesis, was characterized and the structure of a hemopexin-type fold among plant proteins illustrated. Crystal structure of LS-24 determined at 2.2 A resolution by multiple isomorphous replacement phasing showed four-bladed beta-propeller structure having a pseudo 4-fold molecular symmetry along a metal ion-binding central channel. The structure represents typical mammalian hemopexin fold with discernible features correlated with the possible functional variations. The protein was found to exist in the dimeric state. While LS-24 dimer binds to spermine in the crystal structure as well as in solution, binding of heme in solution resulted in the dissociation of the dimer into monomers with concomitant release of bound spermine. Interactions of heme and spermine with LS-24 bear physiological implications. While binding of spermine to LS-24 can be linked with polyamine biosynthesis that of heme correlates with oxidative stress. Mutually exclusive binding of heme and spermine in different oligomeric states suggest a role for LS-24 in sensing oxidative stress through a ligand-regulated monomer-dimer transition switch.
Highlights
A regulatory protein from grass pea (Lathyrus sativus), LS-24, a close homolog of albumin 2 from garden pea (Pisum sativum) that is associated with polyamine biosynthesis, was characterized and the structure of a hemopexin-type fold among plant proteins illustrated
The sequences of the internal protein fragments of LS-24 obtained by chemical and enzymatic methods and interpreting electron density map at 2.2 Aresolution calculated from multiple isomorphous replacement (MIR) phasing yielded the complete sequence of LS-24 (Supplemental Fig. S1)
In the context of LS-24 homologs being involved in polyamine biosynthetic pathway (Vigeolas et al, 2008) and stress physiology (Huang et al, 2006), the structural correlation of mammalian hemopexins with LS-24 would shed light on the physiological role of this protein
Summary
A regulatory protein from grass pea (Lathyrus sativus), LS-24, a close homolog of albumin 2 from garden pea (Pisum sativum) that is associated with polyamine biosynthesis, was characterized and the structure of a hemopexin-type fold among plant proteins illustrated. LS-24 shows significant sequence identity with albumin 2 from garden pea (Croy et al, 1984; Harris and Croy, 1985; Gruen et al, 1987; Higgins et al, 1987; Jenne, 1991) and chickpea (Cicer arietinum; Kolberg et al, 1983; Vioque et al, 1998) They are associated with metabolic processes being cytoplasmic in localization (Harris and Croy, 1985), in contrast to the other seed storage proteins that are found in membrane-bound vesicles (Larkins and Hurkman, 1978; Okita and Rogers, 1996). Our structural and biochemical data, in light of the known regulatory roles of spermine and heme, implicate LS-24 in sensing oxidative stress as a regulatory switch
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