CD74 is a member of the regulated intramembrane proteolysis-processed protein family.

Quite a few regulatory proteins, including transcription factors, are normally maintained in a dormant state to be activated after internal or environmental cues. Recently, a novel strategy, requiring proteolytic cleavage, was described for the mobilization of dormant transcription factors. These transcription factors are initially synthesized in an inactive form, whereas "nesting" in integral membrane precursor proteins. After a cleavage event, these new active factors are released from the membrane and can migrate into the nucleus to drive regulated gene transcription. This mechanism, regulated intramembrane proteolysis (RIP), controls diverse biological processes in prokaryotes and eukaryotes in response to a variety of signals. The MHC class II chaperone, CD74 (invariant chain, Ii), was previously shown to function as a signaling molecule in several pathways. Recently, we demonstrated that after intramembranal cleavage, the CD74 cytosolic fragment (CD74-ICD) is released and induces activation of transcription mediated by the NF-kappaB p65/RelA homodimer and the B-cell-enriched coactivator, TAF(II)105. Here, we add CD74 to the growing family of RIP-processed proteins. Our studies show that CD74 ectodomain must be processed in the endocytic compartments to allow its intramembrane cleavage that liberates CD74 intracellular domain (CD74-ICD). We demonstrate that CD74-ICD translocates to the nucleus and induces the activation of the p65 member of NF-kappaB in this compartment.

Sprouty fine-tunes EGF signaling through interlinked positive and negative feedback loops.

BACKGROUND: Growth factors and their receptor tyrosine kinases play pivotal roles in development, normal physiology, and pathology. Signal transduction is regulated primarily by receptor endocytosis and degradation in lysosomes ("receptor downregulation"). c-Cbl is an adaptor that modulates this process by recruiting binding partners, such as ubiquitin-conjugating enzymes. The role of another group of adaptors, Sprouty proteins, is less understood; although, studies in insects implicated the founder protein in the negative regulation of several receptor tyrosine kinases. RESULTS: By utilizing transfection of living cells, as well as reconstituted in vitro systems, we identified a dual regulatory mechanism that combines human Sprouty2 and c-Cbl. Upon activation of the receptor for the epidermal growth factor (EGFR), Sprouty2 undergoes phosphorylation at a conserved tyrosine that recruits the Src homology 2 domain of c-Cbl. Subsequently, the flanking RING finger of c-Cbl mediates poly-ubiquitination of Sprouty2, which is followed by proteasomal degradation. Because phosphorylated Sprouty2 sequesters active c-Cbl molecules, it impedes receptor ubiquitination, downregulation, and degradation in lysosomes. This competitive interplay occurs in endosomes, and it regulates the amplitude and longevity of intracellular signals. CONCLUSIONS: Sprouty2 emerges as an inducible antagonist of c-Cbl, and together they set a time window for receptor activation. When incorporated in signaling networks, the coupling of positive (Sprouty) to negative (Cbl) feedback loops can greatly enhance output diversification.

Invariant chain-induced B cell differentiation requires intramembrane proteolytic release of the cytosolic domain.

Immature B cells differentiate in the spleen into mature B cells, a process that is essential for their participation in the immune response. Previously, we showed that the MHC class II chaperone, invariant chain (Ii), controls this differentiation to the mature stage. Ii cytosolic domain-induced B cell maturation involves activation of transcription mediated by the NF-kappaB p65/RelA homodimer and requires the B cell enriched coactivator, TAF(II)105. In this study we show that the cytosolic region of Ii is cleaved within the plane of the membrane to generate a cytosolic fragment, which is essential for NF-kappaB activation and B cell differentiation. Our results suggest that Ii functions as a membrane-bound inactive inducer of NF-kappaB transcription that is activated by intramembrane proteolytic cleavage.