Cep152 interacts with the cryptic Polo-box of Plk4 and is required for Plk4-induced centriole overduplication.

Primary microcephaly is a rare condition in which brain size is substantially diminished without other syndromic abnormalities. Seven autosomal loci have been genetically mapped, and the underlying causal genes have been identified for MCPH1, MCPH3, MCPH5, MCPH6, and MCPH7 but not for MCPH2 or MCPH4. The known genes play roles in mitosis and cell division. We ascertained three families from an Eastern Canadian subpopulation, each with one microcephalic child. Homozygosity analysis in two families using genome-wide dense SNP genotyping supported linkage to the published MCPH4 locus on chromosome 15q21.1. Sequencing of coding exons of candidate genes in the interval identified a nonconservative amino acid change in a highly conserved residue of the centrosomal protein CEP152. The affected children in these two families were both homozygous for this missense variant. The third affected child was compound heterozygous for the missense mutation plus a second, premature-termination mutation truncating a third of the protein and preventing its localization to centrosomes in transfected cells. CEP152 is the putative mammalian ortholog of Drosphila asterless, mutations in which affect mitosis in the fly. Published data from zebrafish are also consistent with a role of CEP152 in centrosome function. By RT-PCR, CEP152 is expressed in the embryonic mouse brain, similar to other MCPH genes. Like some other MCPH genes, CEP152 shows signatures of positive selection in the human lineage. CEP152 is a strong candidate for the causal gene underlying MCPH4 and may be an important gene in the evolution of human brain size.

The Rev protein of human immunodeficiency virus type 1 is an RNA-binding protein that is required for nuclear export of unspliced and partially spliced viral mRNAs. Nuclear import of human immunodeficiency virus type 1 Rev has been suggested to depend on the classic nuclear transport receptor importin ␤, but not on the adapter protein importin ␣. We now show that, similar to importin ␣, Rev is able to dissociate RanGTP from recycling importin ␤, a reaction that leads to the formation of a novel import complex. Besides importin ␤, the transport receptors transportin, importin 5, and importin 7 specifically interact with Rev and promote its nuclear import in digitonin-permeabilized cells. A single arginine-rich nuclear localization sequence of Rev is required for interaction with all importins tested so far. In contrast to the importin ␤-binding domain of importin ␣, Rev interacts with an N-terminal fragment of importin ␤. Transportin contains two independent binding sites for Rev. Hence, the mode of interaction of importin ␤ and transportin with Rev is clearly distinct from that with their classic import cargoes. Taken together, the viral protein takes advantage of multiple cellular transport pathways for its nuclear accumulation.The machinery for transport of macromolecules across the nuclear envelope consists of the nuclear pore complex, which is embedded between the inner and outer nuclear membrane (1), and a large variety of soluble transport factors (for reviews see Refs. 2-4). The transport cargoes are characterized by recognition sequences for soluble transport receptors: nuclear export sequences mediate the interaction of proteins with exportins, allowing transport out of the nucleus, whereas nuclear localization signals (NLSs) 2 bind to importins, promoting transport of proteins into the nucleus. Importins and exportins, collectively also referred to as karyopherins, belong to the importin ␤ superfamily of transport receptors. They interact not only with their transport cargo but also with proteins of the nuclear pore complex, as well as with the small GTP-binding protein Ran, a factor that plays an important role in the assembly or disassembly of transport complexes (3). Importin ␤, the prototype of this family, was originally identified as the transport receptor that is involved in nuclear import of proteins containing a "classic" NLS, i.e. a short stretch of amino acids enriched in basic residues, forming either a single or a bipartite transport motif. These NLSs do not bind to importin ␤ directly but via an adapter protein, importin ␣. Importin ␣ contains a characteristic importin ␤-binding-(IBB) domain. A ternary complex containing the import cargo, importin ␣, and importin ␤ assembles in the cytoplasm, translocates across the nuclear pore complex, and dissociates in the nucleus upon binding of RanGTP to importin ␤. The import receptor then recycles back to the cytoplasm in a complex with RanGTP. A dedicated exportin, CAS, is used for export of importin ␣ out of the nucleus (5). Hundreds of proteins...