Abstract
Access to nuclear genes in eukaryotes is provided by members of the importin (IMP) superfamily of proteins, which are of α- or β-types, the best understood nuclear import pathway being mediated by a heterodimer of an IMP α and IMP β1. IMP α recognises specific targeting signals on cargo proteins, while IMP β1 mediates passage into, and release within, the nucleus by interacting with other components of the transport machinery, including the monomeric guanine nucleotide binding protein Ran. In this manner, hundreds of different proteins can be targeted specifically into the nucleus in a tightly regulated fashion. The IMP α gene family has expanded during evolution, with only a single IMP α (Srp1p) gene in budding yeast, and three (IMP α1, 2/pendulin and 3) and five (IMP α1, -2, -3, -4 and -6) IMP α genes in Drosophila melanogaster and mouse respectively, which fall into three phylogenetically distinct groups. The fact that IMP α3 and IMP α2 are only present in metazoans implies that they emerged during the evolution of multicellular animals to perform specialised roles in particular cells and tissues. This review describes what is known of the IMP α gene family in mouse and in D. melanogaster, including a comparitive examination of their mRNA expression profiles in a highly differentiated tissue, the testis. The clear implication of their highly regulated synthesis during the course of spermatogenesis is that the different IMP αs have distinct expression patterns during cellular differentiation, implying tissue/cell type-specific roles.
Highlights
In eukaryotic cells, bidirectional transport of molecules between the cytoplasm and nucleus relies on access through the nuclear pore complexes (NPC’s) embedded in the nuclear envelope
All three IMP genes are expressed in D. melanogaster gonads, with IMP 2 and IMP 1 mRNA selectively enriched in the testis relative to the ovary, whilst IMP 3 mRNA is expressed at similar levels in both ovary and testis [31]
As a means of elucidating upon how the IMP genes themselves are regulated, we investigated IMP regulation at the level of transcriptional initiation, by using in silico promoter analysis to investigate potential promoter regions of the IMP genes in the mouse
Summary
Bidirectional transport of molecules between the cytoplasm and nucleus relies on access through the nuclear pore complexes (NPC’s) embedded in the nuclear envelope. All three IMP genes are expressed in D. melanogaster gonads, with IMP 2 and IMP 1 mRNA selectively enriched in the testis relative to the ovary, whilst IMP 3 mRNA is expressed at similar levels in both ovary and testis [31] In embryos, both dIMP 1 and dIMP 2 localise to the nucleus at the onset of mitosis, suggesting that dIMP 1 and 2 are required for the import of proteins in mitotically active cells rather than acting generally as a protein importers in all cell types [36, 38]. Spermatogenesis encompasses the significant cellular processes of both mitosis and meiosis with considerable morphological transformations occurring as the diploid cell develops into a haploid, motile cell with a highly condensed nucleus [42, 43] These features make the testis a useful model for studying the involvement of IMP s in developmentally stage-specific roles, considering that all the IMP isoforms of both species are expressed in their respective testis [10, 38]. It has been proposed that the regulated expression of nuclear transport factors and specific nuclear proteins during spermatogenesis may mediate the developmental switches that underlie germ cell differentiation [10, 44, 45]
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