The increasing functional repertoire of U1 snRNA

Splicing is a key process for mRNA maturation, particularly in higher eukaryotes where most protein-coding transcripts contain multiple introns. It is achieved by the concerted action of five snRNAs (small nuclear RNAs) and hundreds of accessory proteins that form the spliceosome. Although snRNAs are present in equal amounts in the spliceosome, there is an overall excess of U1 in human cells. This finding led to the opinion that U1 might be involved in processes other than splicing. Research has shown that this is indeed the case and some examples found from studies in human cell systems are described briefly in the present review. U1 snRNP (small nuclear ribonucleoprotein) complex Human U1 snRNA (small nuclear RNA) is 164 nt long with a well-defined structure consisting of four stem–loops [1,2]. It is normally part of an RNP (ribonucleoprotein) complex (termed U1 snRNP) containing seven Sm proteins as well as U1A, U1C and U1-70k [3]. The conventional function for U1 snRNA is in splicing [4]. Specifically, U1 is require