CAML mediates survival of Myc-induced lymphoma cells independent of tail-anchored protein insertion

Lessons from Using Genetically Engineered Mouse Models of MYC-Induced Lymphoma

Oncogenic overexpression of MYC leads to the fatal deregulation of signaling pathways, cellular metabolism, and cell growth. MYC rearrangements are found frequently among non-Hodgkin B-cell lymphomas enforcing MYC overexpression. Genetically engineered mouse models (GEMMs) were developed to understand MYC-induced B-cell lymphomagenesis. Here, we highlight the advantages of using Eµ-Myc transgenic mice. We thoroughly compiled the available literature to discuss common challenges when using such mouse models. Furthermore, we give an overview of pathways affected by MYC based on knowledge gained from the use of GEMMs. We identified top regulators of MYC-induced lymphomagenesis, including some candidates that are not pharmacologically targeted yet.

The Molecular Biodiversity of Protein Targeting and Protein Transport Related to the Endoplasmic Reticulum

Looking at the variety of the thousands of different polypeptides that have been focused on in the research on the endoplasmic reticulum from the last five decades taught us one humble lesson: no one size fits all. Cells use an impressive array of components to enable the safe transport of protein cargo from the cytosolic ribosomes to the endoplasmic reticulum. Safety during the transit is warranted by the interplay of cytosolic chaperones, membrane receptors, and protein translocases that together form functional networks and serve as protein targeting and translocation routes. While two targeting routes to the endoplasmic reticulum, SRP (signal recognition particle) and GET (guided entry of tail-anchored proteins), prefer targeting determinants at the N- and C-terminus of the cargo polypeptide, respectively, the recently discovered SND (SRP-independent) route seems to preferentially cater for cargos with non-generic targeting signals that are less hydrophobic or more distant from the termini. With an emphasis on targeting routes and protein translocases, we will discuss those functional networks that drive efficient protein topogenesis and shed light on their redundant and dynamic nature in health and disease.

Differential Modes of Orphan Subunit Recognition for the WRB/CAML Complex

A large proportion of membrane proteins must be assembled into oligomeric complexes for function. How this process occurs is poorly understood, but it is clear that complex assembly must be tightly regulated to avoid accumulation of orphan subunits with potential cytotoxic effects. We interrogated assembly in mammalian cells by using the WRB/CAML complex, an essential insertase for tail-anchored proteins in the endoplasmic reticulum (ER), as a model system. Our data suggest that the stability of each subunit is differentially regulated. In WRB’s absence, CAML folds incorrectly, causing aberrant exposure of a hydrophobic transmembrane domain to the ER lumen. When present, WRB can correct the topology of CAML both in vitro and in cells. In contrast, WRB can independently fold correctly but is still degraded in the absence of CAML. We therefore propose that there are at least two distinct regulatory pathways for the surveillance of orphan subunits in the mammalian ER.

Most membrane proteins assemble into multi-subunit complexes. How unassembled subunits are recognized and triaged for degradation is poorly understood. Inglis et al. use the WRB/CAML complex to define two modes of orphan recognition: CAML folds incorrectly without WRB, exposing a degron, while WRB inserts correctly but is degraded when unassembled.

Searching for remote homologs of CAML among eukaryotes

The tryptophan rich basic protein/calcium signal-modulating cyclophilin ligand (WRB/CAML) and Get1p/Get2p complexes, in vertebrates and yeast, respectively, mediate the final step of tail-anchored protein insertion into the endoplasmic reticulum membrane via the Get pathway. While WRB appears to exist in all eukaryotes, CAML homologs were previously recognized only among chordates, raising the question as to how CAML's function is performed in other phyla. Furthermore, whereas WRB was recognized as the metazoan homolog of Get1, CAML and Get2, although functionally equivalent, were not considered to be homologous. CAML contains an N-terminal basic, TRC40/Get3-interacting, region, three transmembrane segments near the C-terminus, and a poorly conserved region between these domains. Here, I searched the NCBI protein database for remote CAML homologs in all eukaryotes, using position-specific iterated-basic local alignment search tool, with the C-terminal, the N-terminal or the full-length sequence of human CAML as query. The N-terminal basic region and full-length CAML retrieved homologs among metazoa, plants and fungi. In the latter group several hits were annotated as GET2. The C-terminal query did not return entries outside of the animal kingdom, but did retrieve over one hundred invertebrate metazoan CAML-like proteins, which all conserved the N-terminal TRC40-binding domain. The results indicate that CAML homologs exist throughout the eukaryotic domain of life, and suggest that metazoan CAML and yeast GET2 share a common evolutionary origin. They further reveal a tight link between the particular features of the metazoan membrane-anchoring domain and the TRC40-interacting region. The list of sequences presented here should provide a useful resource for future studies addressing structure-function relationships in CAML proteins.

The Ways of Tails: the GET Pathway and more

Due to their topology tail-anchored (TA) proteins must target to the membrane independently of the co-translational route defined by the signal sequence recognition particle (SRP), its receptor and the translocon Sec61. More than a decade of work has extensively characterized a highly conserved pathway, the yeast GET or mammalian TRC40 pathway, which is capable of countering the biogenetic challenge posed by the C-terminal TA anchor. In this review we briefly summarize current models of this targeting route and focus on emerging aspects such as the intricate interplay with the proteostatic network of cells and with other targeting pathways. Importantly, we consider the lessons provided by the in vivo analysis of the pathway in different model organisms and by the consideration of its full client spectrum in more recent studies. This analysis of the state of the field highlights directions in which the current models may be experimentally probed and conceptually extended.

The WRB Subunit of the Get3 Receptor is Required for the Correct Integration of its Partner CAML into the ER

Calcium-modulating cyclophilin ligand (CAML), together with Tryptophan rich basic protein (WRB, Get1 in yeast), constitutes the mammalian receptor for the Transmembrane Recognition Complex subunit of 40 kDa (TRC40, Get3 in yeast), a cytosolic ATPase with a central role in the post-translational targeting pathway of tail-anchored (TA) proteins to the endoplasmic reticulum (ER) membrane. CAML has also been implicated in other cell-specific processes, notably in immune cell survival, and has been found in molar excess over WRB in different cell types. Notwithstanding the stoichiometric imbalance, WRB and CAML depend strictly on each other for expression. Here, we investigated the mechanism by which WRB impacts CAML levels. We demonstrate that CAML, generated in the presence of sufficient WRB levels, is inserted into the ER membrane with three transmembrane segments (TMs) in its C-terminal region. By contrast, without sufficient levels of WRB, CAML fails to adopt this topology, and is instead incompletely integrated to generate two aberrant topoforms; these congregate in ER-associated clusters and are degraded by the proteasome. Our results suggest that WRB, a member of the recently proposed Oxa1 superfamily, acts catalytically to assist the topogenesis of CAML and may have wider functions in membrane biogenesis than previously appreciated.

TMUB1 Inhibits BRL-3A Hepatocyte Proliferation by Interfering with the Binding of CAML to Cyclophilin B through its TM1 Hydrophobic Domain

Transmembrane and ubiquitin-like domain-containing 1 (Tmub1) encodes a protein (TMUB1) containing an ubiquitin-like domain and plays a negative regulatory role during hepatocyte proliferation, but its mechanism in this process is still unknown. Here, TMUB1 interfered with the binding of calcium-modulating cyclophilin ligand (CAML) to cyclophilin B, which may represent a key role in the negative regulatory process of TMUB1 in hepatocyte proliferation. Co-immunoprecipitation assays in rat BRL-3A cells confirmed the interaction between TMUB1 and CAML; significant regulation of the influx of Ca2+ ([Ca2+]i) and hepatocyte proliferation occurred following TMUB1 overexpression or knockout. Deletion of the TM1 hydrophobic domain of TMUB1 completely abolished this interaction and led to loss of TMUB1's regulatory effects on cytological behavior. Furthermore, overexpression of TMUB1 completely abolished the interaction between CAML and its downstream protein cyclophilin B, which can act upstream of calcineurin by increasing [Ca2+]i during cell proliferation. Taken together, our results indicate that TMUB1 regulates BRL-3A hepatocyte proliferation by interacting with CAML and further interferes with the binding of CAML to cyclophilin B to decrease cellular [Ca2+]i.