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.