Orthologs in Arabidopsis thaliana of the Hsp70 interacting protein Hip

The Hsp70-interacting protein Hip binds to the adenosine triphosphatase dom ain of Hsp70, stabilizing it in the adenosine 5'-diphosphate-ligated confor mation and promoting binding of target polypeptides. In mammalian cells, Hi p is a component of the cytoplasmic chaperone heterocomplex that regulates signal transduction via interaction with hormone receptors and protein kina ses. Analysis of the complete genome sequence of the model flowering plant Arabidopsis thaliana revealed 2 genes encoding Hip orthologs. The deduced s equence of AtHip-1 consists of 441 amino acid residues and is 42% identical to human Hip. AtHip-1 contains the same functional domains characterized i n mammalian Hip, including an N-terminal dimerization domain, an acidic dom ain, 3 tetratricopeptide repeats flanked by a highly charged region, a seri es of degenerate GGMP repeats, and a C-terminal region similar to the Sti1/ Hop/ p60 protein. The deduced amino acid sequence of AtHip-2 consists of 38 0 amino acid residues. AtHip-2 consists of a truncated Hip-like domain that is 46% identical to human Hip, followed by a C-terminal domain related to thioredoxin. AtHip-2 is 63% identical to another Hip-thioredoxin protein re cently identified in Vitis labrusca (grape). The truncated Hip domain in At Hip-2 includes the amino terminus, the acidic domain, and tetratricopeptide repeats with flanking charged region. Analyses of expressed sequence tag d atabases indicate that both AtHip-1 and AtHip-2 are expressed in A thaliana and that orthologs of Hip are also expressed widely in other plants. The s imilarity between AtHip-1 and its mammalian orthologs is consistent with a similar role in plant cells. The sequence of AtHip-2 suggests the possibili ty of additional unique chaperone functions.