RESUMO
The 20S U5 small nuclear ribonucleoprotein particle (snRNP) is a 17-subunit RNA-protein complex and a precursor of the U4/U6.U5 tri-snRNP, the major building block of the precatalytic spliceosome. CD2BP2 is a hallmark protein of the 20S U5 snRNP, absent from the mature tri-snRNP. Here we report a high-resolution cryogenic electron microscopy structure of the 20S U5 snRNP, shedding light on the mutually exclusive interfaces utilized during tri-snRNP assembly and the role of the CD2BP2 in facilitating this process.
Assuntos
Microscopia Crioeletrônica , Modelos Moleculares , Ribonucleoproteína Nuclear Pequena U5 , Humanos , Ribonucleoproteína Nuclear Pequena U5/química , Ribonucleoproteína Nuclear Pequena U5/metabolismo , Spliceossomos/metabolismo , Spliceossomos/química , Spliceossomos/ultraestrutura , Conformação Proteica , Chaperonas Moleculares/metabolismo , Chaperonas Moleculares/químicaRESUMO
Two small Ras-like GTPases, MglA and SofG, work in synchrony to drive cell polarity and motility in the soil bacterium, Myxococcus xanthus. While MglA regulates two types of motility in Myxococcus and drives cell polarity reversals, SofG regulates social motility enabled by the type IV pili (T4P) machinery. In order to understand the molecular basis of how multiple GTPases act concertedly, we initiated biochemical studies on SofG. A construct of SofG (SofG∆60 ) was purified as a homogenous monomer and could bind to GDP and GTP. Intrinsic GTP hydrolysis by SofG∆60 was negligible. Earlier work from the laboratory revealed that MglB functions both as a GTPase-activating protein (GAP) and a guanine nucleotide exchange factor (GEF) for MglA. Biochemical assays of SofG∆60 established that MglB interacts with GTP-bound SofG∆60 and acts as a GAP for SofG∆60 . Interaction of MglB with SofG∆60 in the GDP-bound conformation was not observed, thereby suggesting that MglB might not act as a GEF for SofG∆60 . The existence of a common GAP for both SofG and MglA could potentially contribute to concerted regulation of their GTPase activities, and mediate crosstalk between the two GTPases involved in motility of M. xanthus. Sequence analysis revealed the features for a SofG-like subclass of prokaryotic small Ras-like GTPases that enable MglB to act as a dual-specificity GAP.