The expression of the SmB and SmB' spliceosome proteins in a variety of cell types and tissues has been investigated. Although SmB is found in all cells studied, the StuB' protein is found only in a small number of rodent cell types. The presence of this protein is correlated with the ability to utilize an alternative pathway of RNA splicing which is not available in most cell types. This is the first demonsIration of tissue specific expression of a protein component of the spliceosome and suggests a role for SmB' in the regulation of some cases of alternative RNA splicing.RNA splicing alternative; Autoimmuno antigen; Tissue-specific expression

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A cDNA clone for the human SmB and B' auto-immune antigens has been isolated by antibody screening of a eDNA expression library. The eDNA clone hybridises with two distinct mRNAs, one of which is expressed in a tissue-specific manner. A fusion protein expressed from the eDNA clone was recognised by a number of sera from systemic lupus erythematosus (SLE) patients containing anti-Sin antibodies but not by sera reactive with other auto-immune antigens. The potential use of this clone in a diagnostic assay for SLE and in elucidating the processes regulating the expression of SmB and B' is discussed.

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The SmN protein is a component of small nuclear ribonucleoprotein particles and is closely related to the ubiquitous SmB and B' splicing proteins. It is expressed in a limited range of tissues and cell types, including several undifferentiated embryonal carcinoma cell lines and undifferentiated embryonic stem cells. The protein declines to undetectable levels when embryonal carcinoma or embryonic stem cells are induced to differentiate, producing primitive endoderm or parietal endoderm or yielding embryonal bodies. This decline is due to a corresponding decrease in the level of the SmN mRNA. The potential role of SmN in the regulation of alternative splicing in embryonic cel lines and early embryos is discussed.The earliest cellular differentiation events of mammalian embryos can be mimicked in vitro with embryonal carcinoma (EC) cell lines derived from murine teratocarcinomas (30). The undifferentiated cells derived from the pluripotential stem cells of these tumors have many properties in common with cells of the inner cell mass of early blastocysts (19). Moreover, several EC cell lines of this type, although capable of indefinite growth in culture as undifferentiated cells, can be induced to differentiate in vitro into a diversity of cell types under different growth conditions (23). For example, the F9 cell line (31) can be induced by suitable treatments to mimic the first differentiation events of the embryonic inner cell mass, in which cells differentiate into parietal or visceral endoderm (13,32).Because of the limited amount of material available from early embryos themselves, EC cell lines have been widely used to study the processes regulating embryonic development. Thus, a number of studies have found changes in the expression of particular proteins in EC cell differentiation. Proteins whose expression changes include those involved in structural (2, 32), enzymatic (31, 32), and cell surface (8,12) functions. Such changes in the expression of particular proteins are likely to reflect corresponding alterations in the activity of cellular regulatory proteins which modulate the expression of these genes. In this regard it is of interest that alterations in the expression of genes encoding several different types of transcription factors, including homeo box-containing proteins (6, 16) and zinc finger-containing proteins (5, 14), and a member of the steroid/thyroid receptor gene family (15) have also been observed during EC cell differentiation. Such changes presumably produce transcriptional induction or repression of the genes regulated by these proteins during EC cell differentiation.Interestingly, however, some evidence suggests that gene regulation may also be modulated at the level of RNA splicing during EC cell differentiation. Thus, early studies (25, 26) suggesting that undifferentiated EC cells and their differentiated derivatives differ in their ability to splice the early RNA produced by the eucaryotic virus simian virus 40 have now been supplemented by a report of differential * Corresponding...