Interaction of synthetic peptides corresponding to the scaffolding domain of Caveolin-3 with model membranes

Secondary structure of caveolins: a mini review

Caveolae are 50–100 nm invaginations found within the plasma membrane of cells. Caveolae are involved in many processes that are essential for homeostasis, most notably endocytosis, mechano-protection, and signal transduction. Within these invaginations, the most important proteins are caveolins, which in addition to participating in the aforementioned processes are structural proteins responsible for caveolae biogenesis. When caveolin is misregulated or mutated, many disease states can arise which include muscular dystrophy, cancers, and heart disease. Unlike most integral membrane proteins, caveolin does not have a transmembrane orientation; instead, it is postulated to adopt an unusual topography where both the N- and C-termini lie on the cytoplasmic side of the membrane, and the hydrophobic span adopts an intramembrane loop conformation. While knowledge concerning the biology of caveolin has progressed apace, fundamental structural information has proven more difficult to obtain. In this mini-review, we curate as well as critically assess the structural data that have been obtained on caveolins to date in order to build a robust and compelling model of the caveolin secondary structure.

The Caveolin-3 G56S sequence variant of unknown significance: Muscle biopsy findings and functional cell biological analysis

Purpose

In the era of next‐generation sequencing, we are increasingly confronted with sequence variants of unknown significance. This phenomenon is also known for variations in Caveolin‐3 and can complicate the molecular diagnosis of the disease. Here, we aimed to study the ambiguous character of the G56S Caveolin‐3 variant.

Experimental design

A comprehensive approach combining genetic and morphological studies of muscle derived from carriers of the G56S Caveolin‐3 variant were carried out and linked to biochemical assays (including phosphoblot studies and proteome profiling) and morphological investigations of cultured myoblasts.

Results

Muscles showed moderate chronic myopathic changes in all carriers of the variant. Myogenic RCMH cells expressing the G56S Caveolin‐3 protein presented irregular Caveolin‐3 deposits within the Golgi in addition to a regular localization of the protein to the plasma membrane. This result was associated with abnormal findings on the ultra‐structural level. Phosphoblot studies revealed that G56S affects EGFR‐signaling. Proteomic profiling demonstrated alterations in levels of physiologically relevant proteins which are indicative for antagonization of G56S Caveolin‐3 expression. Remarkably, some proteomic alterations were enhanced by osmotic/mechanical stress.

Conclusions and clinical relevance

Our studies suggest that G56S might influence the manifestation of myopathic changes upon the presence of additional cellular stress burden. Results of our studies moreover improve the current understanding of (genetic) causes of myopathic disorders classified as caveolinopathies.

Recent progress in the topology, structure, and oligomerization of caveolin: a building block of caveolae

Caveolae are cholesterol-rich plasma membrane invaginations that are found in a plethora of cell types. They play many roles including signal transduction, endocytosis, and mechanoprotection. The most critical protein in caveolae is the integral membrane protein, caveolin, which has been shown to be necessary for caveolae formation, and governs the major functions attributed to caveolae. Caveolin is postulated to act as a scaffold in the high molecular weight striated coat that surrounds the caveolar bulb, stabilizing it. Caveolin interacts, both directly and indirectly, with a large number of signaling molecules, and presides over the endocytosis of molecular cargo by caveolae. However, many of the key biophysical aspects of the caveolin protein, its structure, topology, and oligomeric behavior, are just beginning to come to light. Herein is an up-to-date summary and critique of the progress that has been made in understanding caveolin on a molecular and atomic level.

Structural and dynamic properties of juxta-membrane segments of caveolin-1 and caveolin-2 at the membrane interface

Caveolins (cav1-3) are essential membrane proteins found in caveolae. The caveolin scaffolding domain of cav-1 includes a short sequence containing a CRAC motif (V94TKYWFYR101) at its C-terminal end. To investigate the role of this motif in the caveolin-membrane interaction at the atomic level, we performed a detailed structural and dynamics characterization of a cav-1(V94-L102) nonapeptide encompassing this motif and including the first residue of cav-1 hydrophobic domain (L102), in dodecylmaltoside (DM) or dodecylphosphocholine (DPC) micelles, as membrane mimics. Cav-1(V94-L102) partitioned better in DPC and in DM/anionic lipid micelles than in DM micelles, as shown by fluorescence titration and CD. NMR data revealed that this peptide folded as an amphipathic helix located in the polar head group region of DPC micelles. The two tyrosine side-chains, flanked by arginine and lysine residues, are situated on one face of this helix, whereas the phenylalanine and tryptophan side-chains are located on the opposite face. Fluorescence studies showed significant Trp subnanosecond rotations, the presence of several rotamers, and a heterogeneous location within the water/micelle interface. NMR studies of the shorter cav-1(V94-R101) peptide and of the homologous sequence of cav-2(I79SKYVMYKF87) allowed the description of the effect of L102 and of the amino acid variations occurring in cav-2 on the structure and localization in DPC micelles. Based on the topological model of caveolins, our results suggest that the cav-1 and cav-2 nonapeptides studied form interfacial alpha-helix membrane anchors in which the K/RhhhYK/Rh motif, also found in cav-3, may play a significant role.