Cell growth in response to nutrient availability is controlled by the mammalian target of rapamycin complex 1 (mTORC1) kinase pathway, which involves the dynamic assembly of large multiprotein signalling complexes at the lysosomal membrane. Under favourable growth conditions, Rag GTPase heterodimers and associated regulatory factors recruit mTORC1 kinase to the lysosome, activating pro-growth pathways. As a fundamental growth switch, mTORC1 and its regulators represent high value drug targets which have successfully been exploited in oncology and immunology settings. Despite decades of focussed research on mTOR signalling, new components of the pathway continue to be discovered, expanding our understanding of fundamental biology and the therapeutic potential of this pathway.
GPR137b, a largely unstudied lysosomal G protein-coupled receptor (GPCR)-like protein, was identified as a new activator of mTORC1 signalling (Gan et al, 2019). By increasing recruitment and activation of Rag GTPases at the lysosomal membrane in the presence of abundant amino acids, GPR137b stimulates mTORC1 activation and has been highlighted as a promising therapeutic target for mTORC1 dependent cancers.
In this project we aim to characterise the structure and function of GPR137b at the lysosomal membrane and examine the molecular mechanism by which it regulates mTORC1 signalling. We were able to express, solubilise and purify recombinant GPR137b from a mammalian expression system, and show that it is recruited to the lysosomal membrane through a specific interaction with the lysosomal resident glycoprotein LAMP1. The relatively small size of the GPR137b-LAMP1 complex and the lack of structured asymmetry outside of the detergent micelle makes this a difficult target for structural determination by cryo-EM. Using an engineered LAMP1-BRIL fusion protein, we were able to append an anti-BRIL FAb fragment, and anti-FAb nanobody to increase particle size and asymmetry (Xie et al, 2025). This fiducial-assisted approach allowed us to generate distinct 2D classes of a binary GPR137b:LAMP1 complex and forms the basis for ongoing high resolution structure determination. Using proximity labelling approaches, we seek to define additional functional interactions that GPR137b makes at the lysosomal membrane under different nutrient availability conditions and characterise the mechanism by which GPR137b recruits RAG GTPases to activate mTORC1 signalling.