Caracterización de las proteínas tim com receptores de fofatidilserina

Resumen

The T-cell/Transmembrane immunoglobulin and mucin domain (TIM) proteins play an important role in the regulation of the innate and adaptive immune responses. Structures of the N-terminal domain of the murine TIM receptors revealed a conserved immunoglobulin (Ig) type V fold (IgV), with four Cys residues bridging a distinctive CC¿ loop to the GFC ß-sheet. Crystal structures of murine IgV domain of mTIM-4 identify a metal-ion-dependent ligand binding site (MILIBS) conserved in all TIM proteins, except in mTIM-2. The characteristic CC¿ and FG loops of TIM IgV domain shape a narrow cavity where acidic compounds penetrate and coordinate to a metal ion bound to conserved residues in TIM proteins. The structure of the IgV domains of mTIM-4 and mTIM-3 bound to phosphatidylserine (PS) show that the hydrophilic head penetrates into the MILIBS and coordinates with the metal ion, whereas the hydrophobic residues lining the pocket walls interact with the apolar phospholipid moiety. Mutation of protein residues engaged in metal ion coordination or building up the MILIBS in the IgV domain decreases mTIM PS binding, proving the relevance of this site for ligand recognition. Biophysical studies show that the MILIBS walls penetrate into membrane surfaces containing FS. A model for TIM protein binding to FS in membranes is provided. Polymorphisms of TIM proteins have been related to susceptibility to various atopic diseases, particularly asthma. In this work we show that TIM polymorphisms play a role in PS binding by TIM proteins. The TIM-1 mucin polymorphic variants differ in PS binding when TIM-1 protein is expressed on the cell surface. Moreover, mTIM-3 polymorphic variants present FS binding differences due to distinct contacts with the membrane surface. Therefore, susceptibility to atopic diseases might be linked to distinct affinities of the TIM proteins for FS. Research included in this Thesis demonstrated that the TIM proteins (TIM-1, TIM-3 and TIM-4) are pattern recognition receptors specialized in the recognition of PS, a cell death signal. PS is exposed on the outer membrane when a cell undergoes apoptosis, being recognized by TIM proteins. Recognition of apoptotic cells can lead to either cell engulfment (TIM-3 and TIM-4), a key biological process for maintenance of tissue homeostasis and prevention of autoimmunity, or cell stimulation (TIM-1 and TIM-3). Therefore TIM proteins are a functional repertoire for recognition of apoptotic cells, a process that can lead to immune activation or tolerance, depending on the TIM molecule and cell on which it is expressed.