MGRN1 and ATRN cooperate to regulate melanocortin signaling

Proteins perform nearly every function in a cell. It seems intuitive that cells are constantly making proteins so they can perform the tasks necessary to sustain life. Less intuitive, perhaps, is that some of these proteins will inevitably end up in the cellular garbage.

So, why would a cell make proteins just to have them end up in the garbage? This can happen for several reasons. For one, a protein can become damaged and unable to do its job. Having it around the cell is a waste of space, just like how having a broken plate is a waste of space in your cupboard. In other cases, the cell can mistakenly make too much of one type of protein, like how you can buy too much produce that rots before you can cook with it. Other times, the cell simply does not need a particular job done right now, so the protein gets thrown out, like how you swore you’d become an expert pasta maker, but now the tools to do it just take up space. Cells depend on a complex network of other proteins to determine what proteins should be thrown out, and to tag and degrade these proteins.

MGRN1 is one member of the network of proteins responsible for handling the cellular garbage. It belongs to a family of proteins known as E3 ubiquitin ligases. There are hundreds of known E3 ubiquitin ligases, and they each attach ubiquitin to a specific set of protein targets. After the ubiquitin is attached, other cell machinery finds the tagged protein and degrades it. While MGRN1 has been well characterized as an E3 ubiquitin ligase, its precise protein targets have remained more elusive. Previous work from Dr. Jenn Kong, a professor in the Biochemistry Department at the University of Washington, has demonstrated that MGRN1 targets MEGF8, one type of membrane protein adapter, for degradation to regulate a major developmental signaling pathway. However, mice lacking MGRN1 have severe outcomes including heart malformations, craniofacial abnormalities, pigmentation defects, neurodegeneration, and mitochondrial dysfunction, suggesting that MGRN1 has other important targets as well.

To find other targets of MGRN1, Dr. Kong and her team used mass spectrometry to identify proteins that physically interact with MGRN1. They identified nine previously unknown MGRN1-interacting proteins. Because MGRN1 is known to regulate membrane signaling proteins, the team chose to focus on the two membrane-associate proteins that came up in their screen: ATRN and ATRNL.

ATRN and ATRNL are adapter proteins that facilitate interactions between other proteins, but direct evidence for interactions between the two proteins and MGRN1 had not been reported before. To identify regions on ATRN1 and ATRNL1 that could be facilitating their MGRN1 interactions, the team compared their protein sequences to that of MEGF8, a known MGRN1 interactor. They found that the proteins shared significant sequence homology in their cytoplasmic portion, indicating that this region is likely important for their interaction with MGRN1. The group next generated several mutant versions of the MGRN1 protein missing small portions of the cytoplasmic domain. They found that the mutant MGRN1 lacking the RING (short for Really Interesting New Gene) domain lost its ability to interact with ATRN. This result was somewhat unexpected – RING domains canonically perform the ubiquitin transfer for E3 ligases. “It is unusual…but when we finally saw the structure, everything made more sense,” says Dr. Kong.

Knowing that MGRN1 interacts with ATRN1 and ATRNL1 is an important step forward, but ATRN1 and ATRN1L are adapter proteins, suggesting that they facilitate interactions between MGRN1 and other proteins. So, which proteins are targeted for degradation by the MGRN1 and ATRN complex? Previous mouse experiments have shown that MGRN1 and ATRN inhibit melanocortin signaling through the MC1R and MC4R receptors. The group hypothesized that that MGRN1 and ATRN might interact to degrade MC1R and MC4R and decrease melanocortin signaling. They found that when MGRN1 and ATRN were coexpressed, MC4R was highly ubiquitylated and downstream signaling from this receptor was reduced. When only MGRN1 was expressed, there was no change in MC4R ubiquitylation or signaling. These results indicate that ATRN is a crucial adapter for MGRN1 to target MC4R for degradation and decrease melanocortin signaling. Melanocortin receptors regulate pigmentation, energy metabolism, and inflammation, and the MGRN1-ATRN interaction with melanocortin receptors is a new insight into how all of these pathways are regulated.

In the future, Dr. Kong hopes that her team can continue characterizing other proteins targeted by MGRN1. This work will reveal fundamental aspects of MGRN1 biology, and it also has the potential to be leveraged as a tool. “Ultimately, if we can better understand MGRN1 biology and how it targets receptors, we would love to start to explore in the area of targeted protein degradation,” says Kong.

Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium Member Dr. Jennifer Kong contributed to this research.

This work was supported by funding from the National Institutes of Health, Dick and Anne Schneider, the McLaughlin Research Institute, the Howard Hughes Medical Institute Hanna H. Gray Fellows Program, the University of Washington Institute of Stem Cell and Regenerative Medecine, Cancer Research UK, Wellcome Trust, and Open Access funding from the University of Washington.

Parashara P, Gao L, Riglos A, Lartey D, Sidhu SB, Marks T, Williams C, Siauw G, Lee K, Ostrem AIL, Siebold C, Riffle M, Kinnebrew M, Gunn TM, Kong JH. 2025. The E3 ubiquitin ligase MGRN1 targets melanocortin receptors MC1R and MC4R via interactions with transmembrane adapters. J Cell Sci. 138(23):jcs264084. https://doi.org/10.1242/jcs.264084