The present study documents the binding interaction of skeletal muscle 
sarcoplasmic reticulum (SR) transmembrane protein triadin with peripheral 
histidine-rich, Ca(2+)-binding protein (HCP). In addition to providing further 
evidence that HCP coenriches with RyR1, FKBP-12, triadin and calsequestrin (CS) 
in sucrose-density-purified TC vesicles, using specific polyclonal antibody, we 
show it to be expressed as a single protein species, both in fast-twitch and 
slow-twitch fibers, and to identically localize to the I-band. Colocalization of 
HCP and triadin at junctional triads is supported by the overlapping staining 
pattern using monoclonal antibodies to triadin. We show a specific binding 
interaction between digoxigenin-HCP and triadin, using ligand blot techniques. 
The importance of this finding is strengthened by the similarities in binding 
affinity and in Ca2+ dependence, (0.1-1 mM Ca2+) of the interaction of 
digoxigenin-HCP with immobilized TC vesicles. Suggesting that triadin dually 
interacts with HCP and with CS, at distinct sites, we have found that triadin-CS 
interaction in overlays does not require the presence of Ca2+. Consistent with 
the binding of CS to triadin luminal domain (Guo and Campbell, 1995), we show 
that binding sites for digoxigenin-CS, although not binding sites for 
digoxigenin-HCP, can be recovered in the 92 kDa triadin fragment, after 
chymotryptic cleavage of the NH2-terminal end of the folded molecule in intact 
TC vesicles. These differential effects form the basis for the hypothesis that 
HCP anchors to the junctional membrane domain of the SR, through binding to 
triadin short cytoplasmic domain at the NH2 terminus. Although the function of 
this interaction, as such, is not well understood, it seems of potential 
biological interest within the more general context of the structural-functional 
role of triadin at the triadic junction in skeletal muscle.