Cell signalling

A novel Epac-specific cAMP analogue demonstrates independent regulation of Rap1 and ERK. Enserink, J. M. et al. Nature Cell Biol. 4, 901–906 (2002)

Protein kinase A (PKA) is required for the cyclic AMP-induced activation of the extracellular signal-regulated kinase (ERK), but not for the cAMP-induced activation of the small GTPase Rap1 through the guanine nucleotide-exchange factors Epac1 and Epac2. By using a cAMP analogue as a tool to distinguish between PKA- and Epac-mediated effects, the authors found that the cAMP-induced regulation of ERK and the activation of Rap1 are independent processes.

Nuclear transport

Karyopherins in nuclear pore biogenesis: a role for Kap121p in the assembly of Nup53p into nuclear pore complexes. Lusk, C. P. et al. J. Cell Biol. 159, 267–278 (2002)

Karyopherins — also known as importins or exportins, depending on the direction of transport — mediate nuclear transport through the nuclear pore complex (NPC). Karyopherin Kap121 interacts specifically with nucleoporin Nup53, and Lusk et al. now show that Kap121 targets Nup53 to the NPC. Nup53 is subsequently released from Kap121 by Nup170, which allows Kap121 to continue its movement through the NPC.

Telomeres

A bulged stem tethers Est1p to telomerase RNA in budding yeast. Seto, A. G. et al. Genes Dev. 16, 2800–2812 (2002)

The RNA subunit of telomerase functions as the template for telomeric DNA synthesis, but it is not clear whether the RNA subunit has other functions. Here, Seto et al. report the identification of a conserved RNA bulged stem that is essential for telomerase function in vivo and that interacts with the telomerase regulatory protein Est1, which recruits or activates telomerase at the telomere.

Gene expression

The SR protein SRp38 represses splicing in M phase cells Shin, C. & Manley, J. L. Cell 111, 407–417 (2002)

SR proteins function in both constitutive and alternative pre-mRNA splicing. Shin and Manley now report the identification of an unusual member of the family — SRp38. Unlike other SR proteins that induce splicing, SRp38 acts as a splicing repressor that is activated by dephosphorylation during M phase. In mitotic cells splicing is inhibited by SRp38, which implies that cell-cycle-specific dephosphorylation of SRp38 is involved in gene silencing during mitosis.