Structure and Function of Tetraspanin Complexes

Project Summary
Tetraspanins are an ancient and exceptionally well-conserved family of four pass transmembrane proteins –
numbering 33 unique members in humans – that have essential but poorly understood functions in many different
cellular contexts. Among tetraspanin proteins, CD81 and the C8 subfamily stand out as particularly important in
human biology. CD81 is a binding partner of CD19 in the B cell co-receptor complex, a key regulator of B cell
receptor (BCR) signaling. Relatively little is known, however, about how the dynamic association of CD81 with
CD19 regulates its association with the B cell receptor and ability to induce a signaling response, critical
knowledge gaps that impede therapeutic efforts to modulate or target this pathway. The subset of tetraspanins
with eight extracellular cysteine residues (the “C8” tetraspanins) facilitate the maturation and function of the
transmembrane metalloprotease ADAM10, which has numerous important roles in physiology and disease. In
the immune system, ADAM10 promotes T cell proliferation and cytokine production as the primary sheddase of
the low-affinity IgE receptor CD23, and it cleaves Lag-3 in response to stimulation of the T cell receptor. ADAM10
also catalyzes ligand-dependent physiologic Notch cleavage, an essential step in Notch signal transduction, and
is the protease responsible for constitutive alpha secretase processing of the Alzheimer’s precursor protein APP,
resulting in production of a non-toxic product instead of the toxic A-beta (Aβ) fragment produced by beta-
secretase. The objective of this work is to provide a deep and comprehensive understanding of the molecular
basis for the function of CD81 and the C8 tetraspanins using structural, biochemical, and dynamic approaches,
building from our recent progress in visualizing the structure of CD81 in its free and CD19-bound states. To
achieve this goal, we plan to elucidate the dynamics of the CD19-CD81 co-receptor complex in response to B
cell activation, determine the molecular basis for recognition of ADAM10 by C8 tetraspanins, and uncover how
C8-tetraspanin binding to ADAM10 modulates substrate selection and proteolytic activity. Together, successful
completion of these aims will provide a deep understanding of how CD81 modulates signal transduction by the
B cell co-receptor, and how the C8 tetraspanins influence ADAM10 metalloprotease function. These findings will
inform future therapeutic efforts targeting these important biological processes.

Grant Number: 5R01AI172846-04
NIH Institute/Center: NIH
Principal Investigator: Stephen Blacklow