Immune recognition by non-catalytic tyrosine phosphorylated receptors. (360G-Wellcome-101799_Z_13_Z)

Leucocyte interactions with their environment are mediated primarily by a large array of structurally diverse cell surface receptors. The members of one large, functionally important subset are phosphorylated on cytoplasmic tyrosine residues upon ligand engagement. We refer to them as Non-catalytic Tyrosine phosphorylated Receptors, or NTRs. A recent survey has identified over a 100 NTRs on human leucocytes in 16 distinct families (see Table and Figure 1) (Dushek et al., 2012). Although NTRs have no overall sequence similarity, they do share key features that suggest a common signal transduction or triggering mechanism (see Table and Figures 1 and 2). 1. They have small extracellular domains and bind ligands that are also small and surface-anchored. 2. They either have, or associate with adaptor subunits that have, conserved tyrosine-containing motifs in their cytoplasmic domains. 3. These tyrosines are phosphorylated by SRC-family kinases, which are constitutively associated w ith the plasma membrane. 4. This phosphorylation is regulated by receptor protein tyrosine phosphatases (RPTP) with large ectodomains, such as CD45 and CD148, which are widely expressed on all leukocytes. NTRs all use one of a limited number of tyrosine-containing motifs to transduce activatory or inhibitory signals. The main ones are immunotyrosine-based activation motifs (ITAMs) and immunotytosine-based inhibitory motifs (ITIMs). Other motifs include the activatory YxxM motif and the immuno tyrosine-based switch motif (ITSM), which can be either activatory or inhibitory. A striking feature of NTRs is that they directly influence each others signalling close to the membrane through a process that we refer to as membrane-proximal signal integration. Thus, NTRs with inhibitory motifs can block the earliest signalling steps of NTRs with activatory motifs. It has been observed that many NTRs aggregate into clusters following engagement of their ligands. However the mechanism of aggre gation and the functional significance thereof are poorly understood. We propose to address three main research questions: 1. What is the mechanism by which the binding of NTRs to their ligands results in phosphorylation of their cytoplasmic tyrosine residues, a process we refer to as NTR triggering? We will test the hypothesis that they all use the kinetic-segregation (K-S) mechanism for triggering. 2. How do NTRs integrate their signals with each other and with other receptors? 3. What ar e the mechanisms driving, and functional consequences of, NTR aggregation into clusters?