SNF1-related protein kinase (SnRK1) phosphorylates class I heat shock protein

The nucleotide sequence of cBSnIP2, a cDNA that had been cloned from a barley (Hordeum vulgare) seed endosperm cDNA library by two-hybrid screening with barley SNF1-related protein kinase (SnRK1) was determined. It was found to contain a complete open reading frame encoding a class I heat shock protein. Transcripts corresponding to the cDNA (renamed cBHSP17) were detectable in RNA isolated from barley seeds harvested in mid-development but not RNA from roots or leaves. BHSP17 protein was expressed in Escherischia coli and shown to be phosphorylated by SnRK1 from barley endosperm and spinach leaf. It was found to be a less effective substrate than 3-hydroxy-3-methylglutaryl-Coenzyme A, a previously identified substrate of SnRK1. However, a specific phosphorylation site at serine-35 was identified by solid phase sequencing of RP-HPLC-purified peptides after phosphorylation by spinach SnRK1.