In investigating the composition of stored (maternal) mRNP particles in Xenopus oocytes, attention has focussed primarily on the phosphoproteins pp60/56, which are Y-box proteins involved in a general packaging of mRNA. We now identify a third, abundant, integral component of stored mRNP particles, Xp54, which belongs to the family of DEAD-box RNA helicases. Xp54 was first detected by its ability to photocrosslink ATP. Subsequent sequence analysis identifies Xp54 as a member of a helicase subfamily which includes: human p54, encoded at a chromosomal breakpoint in the B-cell lymphoma cell line, RC-K8; Drosophila ME31B, encoded by a maternally-expressed gene, and Saccharomyces pombe Ste13, cloned by complementation of the sterility mutant ste13. Expression studies reveal that the gene encoding Xp54 is transcribed maximally at early oogenesis: no transcripts are detected in adult tissues, other than ovary. Using a monospecific antibody raised against native Xp54, its presence in mRNP particles is confirmed by immunoblotting fractions bound to oligo(dT)-cellulose and separated by rate sedimentation and buoyant density. On isolating Xp54 from mRNP particles, it is shown to possess an ATP-dependent RNA helicase activity. Possible functions of Xp54 are discussed in relation to the assembly and utilization of mRNP particles.

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Members of the Y-box (YB) family of transcription factors are expressed in a wide range of cell types and are implicated in the regulation of a rapidly increasing number of genes. Although the biological activities of YB proteins appear to be varied, distinct patterns, relating to the timing of their expression and the identity of their target genes, are beginning to emerge. A recent report by Ito et al. focusses attention on cell proliferation and adds support to earlier suggestions that a primary function of YB proteins is to help activate growth-associated genes.

The translation of a capped, polyadenylated RNA after injection into the nucleus of Xenopus oocytes occurs only if the RNA contains an intron. A single point mutation in the splice donor site prevents translation. Intron-less RNA is exported efficiently to the cytoplasm and is held, undegraded, in a translationally inert state for several days. Translation can be activated by treating the oocytes with progesterone or by injecting antibodies that bind the FRGY2 class of messenger RNA binding proteins, p56 and p60, but these antibodies are only effective if delivered to the nucleus. Inhibitors of casein kinase 11 also activate translation whereas phosphatase inhibitors block progesteronemediated activation of translation. These data suggest the presence of an RNA handling pathway in the nucleus of Xenopus oocytes which is regulated by casein kinase type 11 phosphorylation and which directs transcripts to be sequestered by p56/p60 or by closely related proteins. This pathway can be bypassed if the RNA contains an intron and it can be reversed by progesterone treatment. These data may have implications for understanding translational control during early development.

The Y-box proteins are defined by their ability to bind to Y-box promoter elements and to help regulate transcription of a wide variety of genes. However, Y-box proteins are also identified as abundant proteins in the cytoplasm of germ cells, where they are found bound to stored mRNA molecules. Binding of Y-box proteins to mRNA sequences, both in vitro and in vivo, has been shown to effect their translational repression ("masking"). Here we discuss the ability of Y box proteins to recognize different nucleic acid structures and to become involved in regulation of both transcription and translation.