Actin dynamics are implicated in various cellular processes, not only through the regulation of cytoskeletal organization, but also via the control of gene expression. In the present study we show that the Src family kinase substrate p130Cas (Cas is Crk-associated substrate) influences actin remodelling and concomitant muscle-specific gene expression, thereby regulating myogenic differentiation. In C2C12 myoblasts, silencing of p130Cas expression by RNA interference impaired F-actin (filamentous actin) formation and nuclear localization of the SRF (serum-response factor) co-activator MAL (megakaryocytic acute leukaemia) following the induction of myogenic differentiation. Consequently, formation of multinucleated myotubes was abolished. Re-introduction of wild-type p130Cas, but not its phosphorylation-defective mutant, into p130Cas-knockdown myoblasts restored F-actin assembly, MAL nuclear localization and myotube formation. Depletion of the adhesion molecule integrin β3, a key regulator of myogenic differentiation as well as actin cytoskeletal organization, attenuated p130Cas phosphorylation and MAL nuclear localization during C2C12 differentiation. Moreover, knockdown of p130Cas led to the activation of the F-actin-severing protein cofilin. The introduction of a dominant-negative mutant of cofilin into p130Cas-knockdown myoblasts restored muscle-specific gene expression and myotube formation. The results of the present study suggest that p130Cas phosphorylation, mediated by integrin β3, facilitates cofilin inactivation and promotes myogenic differentiation through modulating actin cytoskeleton remodelling.

Spezifizieren von 14‐3‐3: Ein fragmentbasierter kombinatorischer Peptid‐Mikroarray erzeugt affinitätsbasierte Fingerprints von sieben Säuger‐14‐3‐3‐Isoformen. Motive mit hoher Affinität für die hoch homologen Isoformen wurden identifiziert. Wahrscheinliche 14‐3‐3σ‐spezifische Peptide wurden zudem durch radiometrisches Zwei‐Farben‐Screening identifiziert (siehe Bild).

Specifying 14‐3‐3: A fragment‐based combinatorial peptide microarray generates affinity‐based fingerprints of seven mammalian 14‐3‐3 isoforms. High‐affinity motifs are identified against the highly homologous isoforms. Putative 14‐3‐3σ‐specific peptides were also delineated by a dual‐color ratiometric screening strategy (see picture).

Methods that allow visualisation of proteins in living systems, in real time have been key to our understanding of the molecular underpinnings of life. Although the use of genetically encoded fusions to fluorescent proteins have greatly advanced such studies, the large size of these tags and their ability to perturb protein activity has been major limitations. Attempts to circumvent these issues have seen the genesis of complementary strategies to chemically label/modify proteins. Thus, chemical labelling approaches seek to "decorate" biomolecules in live cells through the site-specific introduction of a small, non-native chemical tag (or reporter group). The introduced tag is minimally invasive such that the activity and/or function of the target molecule in not perturbed/compromised by its inclusion. In most cases, this modification is brought about by fusing target biomolecules to protein domains/peptide tags or via the incorporation of reactive "handles" by either exploiting the cell's biosynthetic machinery or during protein synthesis. Selective tagging of the biomolecule then proceeds via a bioorthogonal chemical reaction following exogenous addition of probe(s). Depending on the nature of the probe, the method can be applied to either visualise/track the dynamics of target molecule(s) in their native cellular milieu or for affinity enrichment for further downstream applications. The versatility of these approaches has been demonstrated by their ability to tag not just proteins but also intractable biomolecules like lipids and glycans. In this review, we summarise the various strategies available to "chemically" tag proteins and provide a comparative analysis their advantages and disadvantages. We also highlight the many creative applications of such methodologies and discuss their future prospects.

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