Zinc finger-like structure in U1-specific protein C is essential for specific binding to U1 snRNP

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Abstract
The U1 small nuclear ribonucleoprotein (snRNP) contains three specific proteins denoted 70K, A and C, in addition to the common proteins. Specific functions of these proteins are not known although recently protein C was shown to be involved in the binding of U1 snRNP to the 5' splice site of a pre-mRNA. Unlike proteins A and 70K, U1-C lacks an RNA binding domain (RNP-80 motif) and does not appear to bind directly to U1 snRNA. However, at the amino terminal end protein C contains a zinc finger-like structure of the CC-HH type found in transcription factor TF IIIA. Several lines of evidence indicate that the zinc finger-like structure is essential for the binding of protein C to U1 snRNP particles: i) deletion analysis of protein C showed that the N-terminal 45 amino acids are sufficient for binding to U1 snRNPs, ii) modification of the cysteine residues in the N-terminal domain with N-ethylmaleimide and iii) single point mutations of the cysteines and histidines contributing to the putative zinc finger abolished binding of protein C to U1 snRNPs. Interestingly, unlike the proteins U1-A and U1-70K the U1-C protein is unable to bind to naked U1 snRNA. On the other hand it is shown that protein C does not bind to the known protein constituents of the U1 particle without the U1 snRNA being present. These data indicate that the binding of protein C to U1 snRNP is dependent on the presence of both the U1 snRNA and one or more of the U1 snRNP proteins.