Unusual Ca(2+)-calmodulin binding interactions of the microtubule-associated protein F-STOP

Beyond Neuronal Microtubule Stabilization: MAP6 and CRMPS, Two Converging Stories

The development and function of the central nervous system rely on the microtubule (MT) and actin cytoskeletons and their respective effectors. Although the structural role of the cytoskeleton has long been acknowledged in neuronal morphology and activity, it was recently recognized to play the role of a signaling platform. Following this recognition, research into Microtubule Associated Proteins (MAPs) diversified. Indeed, historically, structural MAPs—including MAP1B, MAP2, Tau, and MAP6 (also known as STOP);—were identified and described as MT-binding and -stabilizing proteins. Extensive data obtained over the last 20 years indicated that these structural MAPs could also contribute to a variety of other molecular roles. Among multi-role MAPs, MAP6 provides a striking example illustrating the diverse molecular and cellular properties of MAPs and showing how their functional versatility contributes to the central nervous system. In this review, in addition to MAP6’s effect on microtubules, we describe its impact on the actin cytoskeleton, on neuroreceptor homeostasis, and its involvement in signaling pathways governing neuron development and maturation. We also discuss its roles in synaptic plasticity, brain connectivity, and cognitive abilities, as well as the potential relationships between the integrated brain functions of MAP6 and its molecular activities. In parallel, the Collapsin Response Mediator Proteins (CRMPs) are presented as examples of how other proteins, not initially identified as MAPs, fall into the broader MAP family. These proteins bind MTs as well as exhibiting molecular and cellular properties very similar to MAP6. Finally, we briefly summarize the multiple similarities between other classical structural MAPs and MAP6 or CRMPs.In summary, this review revisits the molecular properties and the cellular and neuronal roles of the classical MAPs, broadening our definition of what constitutes a MAP.

Structural basis for the association of MAP6 protein with microtubules and its regulation by calmodulin

Microtubules are highly dynamic αβ-tubulin polymers. In vitro and in living cells, microtubules are most often cold- and nocodazole-sensitive. When present, the MAP6/STOP family of proteins protects microtubules from cold- and nocodazole-induced depolymerization but the molecular and structure determinants by which these proteins stabilize microtubules remain under debate. We show here that a short protein fragment from MAP6-N, which encompasses its Mn1 and Mn2 modules (MAP6(90-177)), recapitulates the function of the full-length MAP6-N protein toward microtubules, i.e. its ability to stabilize microtubules in vitro and in cultured cells in ice-cold conditions or in the presence of nocodazole. We further show for the first time, using biochemical assays and NMR spectroscopy, that these effects result from the binding of MAP6(90-177) to microtubules with a 1:1 MAP6(90-177):tubulin heterodimer stoichiometry. NMR data demonstrate that the binding of MAP6(90-177) to microtubules involve its two Mn modules but that a single one is also able to interact with microtubules in a closely similar manner. This suggests that the Mn modules represent each a full microtubule binding domain and that MAP6 proteins may stabilize microtubules by bridging tubulin heterodimers from adjacent protofilaments or within a protofilament. Finally, we demonstrate that Ca(2+)-calmodulin competes with microtubules for MAP6(90-177) binding and that the binding mode of MAP6(90-177) to microtubules and Ca(2+)-calmodulin involves a common stretch of amino acid residues on the MAP6(90-177) side. This result accounts for the regulation of microtubule stability in cold condition by Ca(2+)-calmodulin.

Vinflunine: a new vision that may translate into antiangiogenic and antimetastatic activity

Microtubules and tubulin are major dynamic and structural cellular components that play a key role in several cell functions, including division, signalling and intracellular trafficking. Normal epithelial cells have a highly structured, rigid cytoskeletal network that is compatible with cell motility. Thus, tubulin and microtubules are compelling cellular targets for chemotherapy. In fact, among anticancer agents, those that target microtubules constitute one of the most effective classes of chemotherapeutics in cancer. The list of compounds that target either tubulin or microtubules is extensive and consists of chemically unique compounds that bind to the tubulin dimers and destabilize microtubules (Vinca alkaloids) and those that bind to the microtubule polymer and stabilize microtubules (taxanes). Tumour-induced angiogenesis, the formation of new capillaries from existing blood vessels, and epithelial-mesenchymal transition are two steps that are critical for both tumour growth and metastatic spread. Three possible mechanisms of action are described with vinflunine, the new-generation Vinca alkaloid to arrive in clinical practice are as follows: it acts against tubulin and microtubules, disrupts newly formed blood vessels and seems to be able to reduce the metastatic process as shown in preclinical studies. These findings support the hypothesis that vinflunine, by blocking microtubule functions that contribute to cell shape, polarization, migration and other processes, might be responsible not only for tumour-cytostatic but also for specific antiangiogenic or antiepithelial-mesenchymal transition effects.

The C-terminal polyproline-containing region of ELMO contributes to an increase in the life-time of the ELMO-DOCK complex

The eukaryotic Engulfment and CellMotility (ELMO) proteins form an evolutionary conserved family of key regulators which play a central role in Rho-dependent biological processes such as engulfment and cell motility/migration. ELMO proteins interact with a subset of Downstream of Crk (DOCK) family members, a new type of guanine exchange factors (GEF) for Rac and cdc42 GTPases. The physiological function of DOCK is to facilitate actin remodeling, a process which occurs only in presence of ELMO. Several studies have determined that the last 200 C-terminal residues of ELMO1 and the first 180 N-terminal residues of DOCK180 are responsible for the ELMO-DOCK interaction. However, the precise role of the different domains and motifs identified in these regions has remained elusive. Divergent functional, biochemical and structural data have been reported regarding the contribution of the C-terminal end of ELMO, comprising its polyproline motif, and of the DOCK SH3 domain. In the present study, we have investigated the contribution of the C-terminal end of ELMO1 to the interaction between ELMO1 and the SH3 domain of DOCK180 using nuclear magnetic resonance spectroscopy and surface plasmon resonance. Our data presented here demonstrate the ability of the SH3 domain of DOCK180 to interact with ELMO1, regardless of the presence of the polyproline-containing C-terminal end. However, the presence of the polyproline region leads to a significant increase in the half-life of the ELMO1-DOCK180 complex, along with a moderate increase on the affinity.

The calponin regulatory region is intrinsically unstructured: novel insight into actin-calponin and calmodulin-calponin interfaces using NMR spectroscopy

Calponin is an actin- and calmodulin-binding protein believed to regulate the function of actin. Low-resolution studies based on proteolysis established that the recombinant calponin fragment 131-228 contained actin and calmodulin recognition sites but failed to precisely identify the actin-binding determinants. In this study, we used NMR spectroscopy to investigate the structure of this functionally important region of calponin and map its interaction with actin and calmodulin at amino-acid resolution. Our data indicates that the free calponin peptide is largely unstructured in solution, although four short amino-acid stretches corresponding to residues 140-146, 159-165, 189-195, and 199-205 display the propensity to form α-helices. The presence of four sequential transient helices probably provides the conformational malleability needed for the promiscuous nature of this region of calponin. We identified all amino acids involved in actin binding and demonstrated for the first time, to our knowledge, that the N-terminal flanking region of Lys(137)-Tyr(144) is an integral part of the actin-binding site. We have also delineated the second actin-binding site to amino acids Thr(180)-Asp(190). Ca(2+)-calmodulin binding extends beyond the previously identified minimal sequence of 153-163 and includes most amino acids within the stretch 143-165. In addition, we found that calmodulin induces chemical shift perturbations of amino acids 188-190 demonstrating for the first time, to our knowledge, an effect of Ca(2+)-calmodulin on this region. The spatial relationship of the actin and calmodulin contacts as well as the transient α-helical structures within the regulatory region of calponin provides a structural framework for understanding the Ca(2+)-dependent regulation of the actin-calponin interaction by calmodulin.

Modular architecture of Munc13/calmodulin complexes: dual regulation by Ca2+ and possible function in short-term synaptic plasticity

Ca(2+) signalling in neurons through calmodulin (CaM) has a prominent function in regulating synaptic vesicle trafficking, transport, and fusion. Importantly, Ca(2+)-CaM binds a conserved region in the priming proteins Munc13-1 and ubMunc13-2 and thus regulates synaptic neurotransmitter release in neurons in response to residual Ca(2+) signals. We solved the structure of Ca(2+)(4)-CaM in complex with the CaM-binding domain of Munc13-1, which features a novel 1-5-8-26 CaM-binding motif with two separated mobile structural modules, each involving a CaM domain. Photoaffinity labelling data reveal the same modular architecture in the complex with the ubMunc13-2 isoform. The N-module can be dissociated with EGTA to form the half-loaded Munc13/Ca(2+)(2)-CaM complex. The Ca(2+) regulation of these Munc13 isoforms can therefore be explained by the modular nature of the Munc13/Ca(2+)-CaM interactions, where the C-module provides a high-affinity interaction activated at nanomolar [Ca(2+)](i), whereas the N-module acts as a sensor at micromolar [Ca(2+)](i). This Ca(2+)/CaM-binding mode of Munc13 likely constitutes a key molecular correlate of the characteristic Ca(2+)-dependent modulation of short-term synaptic plasticity.

Multiplexed sorting of libraries on libraries: a novel method for empirical protein design by affinity-driven phage enrichment on synthetic peptide arrays

Chemically synthesized peptide arrays on planar cellulose carriers are proposed as libraries of ligands suitable for the multiplexed simultaneous capture of peptide-specific acceptor proteins from a large randomly mutagenized library of acceptor proteins presented on bacteriophage M13 particles. This experimental set-up can be exploited to rapidly screen for individual new, distinct binding partners from two complementary libraries (two-dimensional screening). The technical feasibility of this empirical protein design approach was demonstrated with calmodulin as an aceptor protein using an array of mastoparan variants for multiplexed phage affinity enrichment.