Matthias Grimmler

Cdk-Inhibitory Activity and Stability of p27Kip1 Are Directly Regulated by Oncogenic Tyrosine Kinases

p27Kip1 controls cell proliferation by binding to and regulating the activity of cyclin-dependent kinases (Cdks). Here we show that Cdk inhibition and p27 stability are regulated through direct phosphorylation by tyrosine kinases. A conserved tyrosine residue (Y88) in the Cdk-binding domain of p27 can be phosphorylated by the Src-family kinase Lyn and the oncogene product BCR-ABL. Y88 phosphorylation does not prevent p27 binding to cyclin A/Cdk2. Instead, it causes phosphorylated Y88 and the entire inhibitory 3(10)-helix of p27 to be ejected from the Cdk2 active site, thus restoring partial Cdk activity. Importantly, this allows Y88-phosphorylated p27 to be efficiently phosphorylated on threonine 187 by Cdk2 which in turn promotes its SCF-Skp2-dependent degradation. This direct link between transforming tyrosine kinases and p27 may provide an explanation for Cdk kinase activities observed in p27 complexes and for premature p27 elimination in cells that have been transformed by activated tyrosine kinases.

Nexilin mutations destabilize cardiac Z-disks and lead to dilated cardiomyopathy

Z-disks, the mechanical integration sites of heart and skeletal muscle cells, link anchorage of myofilaments to force reception and processing. The key molecules that enable the Z-disk to persistently withstand the extreme mechanical forces during muscle contraction have not yet been identified. Here we isolated nexilin (encoded by NEXN) as a novel Z-disk protein. Loss of nexilin in zebrafish led to perturbed Z-disk stability and heart failure. To evaluate the role of nexilin in human heart failure, we performed a genetic association study on individuals with dilated cardiomyopathy and found several mutations in NEXN associated with the disease. Nexilin mutation carriers showed the same cardiac Z-disk pathology as observed in nexilin-deficient zebrafish. Expression in zebrafish of nexilin proteins encoded by NEXN mutant alleles induced Z-disk damage and heart failure, demonstrating a dominant-negative effect and confirming the disease-causing nature of these mutations. Increasing mechanical strain aggravated Z-disk damage in nexilin-deficient skeletal muscle, implying a unique role of nexilin in protecting Z-disks from mechanical trauma.

A Comprehensive Interaction Map of the Human Survival of Motor Neuron (SMN) Complex

Assembly of the Sm-class of U-rich small nuclear ribonucleoprotein particles (U snRNPs) is a process facilitated by the macromolecular survival of motor neuron (SMN) complex. This entity promotes the binding of a set of factors, termed LSm/Sm proteins, onto snRNA to form the core structure of these particles. Nine factors, including the SMN protein, the product of the spinal muscular atrophy (SMA) disease gene, Gemins 2-8 and unrip have been identified as the major components of the SMN complex. So far, however, only little is known about the architecture of this complex and the contribution of individual components to its function. Here, we present a comprehensive interaction map of all core components of the SMN complex based upon in vivo and in vitro methods. Our studies reveal a modular composition of the SMN complex with the three proteins SMN, Gemin8, and Gemin7 in its center. Onto this central building block the other components are bound via multiple interactions. Furthermore, by employing a novel assay, we were able to reconstitute the SMN complex from individual components and confirm the interaction map. Interestingly, SMN protein carrying an SMA-causing mutation was severely impaired in formation of the SMN complex. Finally, we show that the peripheral component Gemin5 contributes an essential activity to the SMN complex, most likely the transfer of Sm proteins onto the U snRNA. Collectively, the data presented here provide a basis for the detailed mechanistic and structural analysis of the assembly machinery of U snRNPs.

RioK1, a New Interactor of Protein Arginine Methyltransferase 5 (PRMT5), Competes with pICln for Binding and Modulates PRMT5 Complex Composition and Substrate Specificity

Protein arginine methylation plays a critical role in differential gene expression through modulating protein-protein and protein-DNA/RNA interactions. Although numerous proteins undergo arginine methylation, only limited information is available on how protein arginine methyltransferases (PRMTs) identify their substrates. The human PRMT5 complex consists of PRMT5, WD45/MEP50 (WD repeat domain 45/methylosome protein 50), and pICln and catalyzes the symmetrical arginine dimethylation of its substrate proteins. pICln recruits the spliceosomal Sm proteins to the PRMT5 complex for methylation, which allows their subsequent loading onto snRNA to form small nuclear ribonucleoproteins. To understand how the PRMT5 complex is regulated, we investigated its biochemical composition and identified RioK1 as a novel, stoichiometric component of the PRMT5 complex. We show that RioK1 and pICln bind to PRMT5 in a mutually exclusive fashion. This results in a PRMT5-WD45/MEP50 core structure that either associates with pICln or RioK1 in distinct complexes. Furthermore, we show that RioK1 functions in analogy to pICln as an adapter protein by recruiting the RNA-binding protein nucleolin to the PRMT5 complex for its symmetrical methylation. The exclusive interaction of PRMT5 with either pICln or RioK1 thus provides the first mechanistic insight into how a methyltransferase can distinguish between its substrate proteins.

Phosphorylation regulates the activity of the SMN complex during assembly of spliceosomal U snRNPs

The assembly of spliceosomal U‐rich small nuclear ribonucleoproteins (U snRNPs) is an ATP‐dependent process mediated by the coordinated action of the SMN and the PRMT5 complex. Here, we provide evidence that the activity of this assembly machinery is regulated by means of post‐translational modification. We show that two main components of the SMN/PRMT5 system, namely the survival motor neuron (SMN) protein (reduced levels thereof causing spinal muscular atrophy) and pICln, are phosphorylated in vivo. Both proteins share a previously unknown motif containing either one or two phosphoserines. Alteration of these residues in SMN (serines 28 and 31) significantly impairs the activity of the SMN complex. Despite the presence of SMN in both the nucleus and cytoplasm, we find that only the latter promotes efficient SMN‐mediated U snRNP assembly activity. As cytoplasmic SMN is phosphorylated to a much larger extent, we hypothesize that this modification is a key activator of the SMN complex.

Dephosphorylation of survival motor neurons (SMN) by PPM1G/PP2Cγ governs Cajal body localization and stability of the SMN complex

The survival motor neuron (SMN) complex functions in maturation of uridine-rich small nuclear ribonucleoprotein (RNP) particles. SMN mediates the cytoplasmic assembly of Sm proteins onto uridine-rich small RNAs, and then participates in targeting RNPs to nuclear Cajal bodies (CBs). Recent studies have suggested that phosphorylation might control localization and function of the SMN complex. Here, we show that the nuclear phosphatase PPM1G/PP2Cγ interacts with and dephosphorylates the SMN complex. Small interfering RNA knockdown of PPM1G leads to an altered phosphorylation pattern of SMN and Gemin3, loss of SMN from CBs, and reduced stability of SMN. Accumulation in CBs is restored upon overexpression of catalytically active, but not that of inactive, PPM1G. This demonstrates that PPM1Gs phosphatase activity is necessary to maintain SMN subcellular distribution. Concomitant knockdown of unr interacting protein (unrip), a component implicated in cytoplasmic retention of the SMN complex, also rescues the localization defects. Our data suggest that an interplay between PPM1G and unrip determine compartment-specific phosphorylation patterns, localization, and function of the SMN complex.