Updated project metadata. Glioblastoma (GBM) is characterized by an exceptionally high intratumoral heterogeneity. However, the molecular mechanisms underlying the origin of different GBM cell populations remain unclear. Here we found that the composition of ribosomes of GBM cells in the tumor core and edge differ due to alternative RNA splicing. The acidic pH in the core switches pre-mRNA splicing of the ribosomal gene RPL22L1 toward the RPL22L1b isoform. To elucidate the functions of RPL22L1 isoforms we identified proteins that interact with RPL22L1a and RPL22L1b. First, each isoform was expressed in E. coli and immobilized on magnetic beads. The beads were incubated with GBM cell lysates and proteins bound to the beads were identified using LC-MS/MS.