Table 2.

Peptides identified by the mass mapping procedure and relative to: isolated ICOSL, isolated ICOSL + DTSSP (indicated with CL); OPN/ICOSL complex +DTSSP (indicated with CL); OPN/ICOSL + DTSSP (indicated with CL) following reduction and alkylation treatment (indicated with CAM).

Lysine Residue	Peptides from ICOSL	Peptides from (ICOSL + DTSSP)	Peptides from the complex (OPN/ICOSL + DTSSP)	Peptides from the complex (OPN/ICOSL + DTSSP) REDCAM
N-TERM	19–23 Exp 619.24 Theor 619.28 19–26 Exp 1003.47 Theor 1003.49	19–26(+1CLINTRA) Exp 1177.44 Theor 1177.49 19–26(+1CLH2O) Exp 1195.45 Theor 1195.49	19–26(+1CLINTRA) Exp 1177.44 Theor 1177.49 19–26(+1CLH2O) Exp 1195.45 Theor 1195.49	19–26(+1CL + 1CAM) Exp 1148.47 Theor 1148.49 19–26(+2CL + 2CAM) Exp 1293.47 Theor 1293.49
K23	19–26 Exp 1003.47 Theor 1003.49	19–26(+1CLINTRA) Exp 1177.44 Theor 1177.49 19–26(+1CLH2O) Exp 1195.45 Theor 1195.49	19–26(+1CLINTRA) Exp 1177.44 Theor 1177.49 19–26(+1CLH2O) Exp 1195.45 Theor 1195.49	19–26(+1CL + 1CAM) Exp 1148.47 Theor 1148.49 19–26(+2CL + 2CAM) Exp 1293.47 Theor 1293.49
K60	45–60 Exp 1992.86 Theor 1992.91	45–60 Exp 1992.86 Theor 1992.91 61–79 Exp 2158.01 Theor 2158.06	45–60 Exp 1992.83 Theor 1992.91 61–79 Exp 2158.00 Theor 2158.06	–
K110	100–110 Exp 1317.61 Theor 1317.66	–	–	–
K174	175–194 Exp 2307.13 Theor 2307.12	175–194 Exp 2307.99 Theor 2307.12	–	–
K241	240–252 Exp 1416.67 Theor 1416.71	240–252(+1CLH2O) Exp 1608.70 Theor 1608.71	240–252 Exp 1416.64 Theor 1416.71 240–252(+1CLH2O) Exp 1609.64 Theor 1609.71	240–252 (+1CL + 1CAM) Exp 1562.66 Theor 1562.71
K252	242–264 Exp 2626.16 Theor 2626.22	242–264 (+1CLH2O) Exp 2818.18 Theor 2818.22 253–264 Exp 1470.61 Theor 1470.64	242–264(+1CLH2O) Exp 2818.18 Theor 2818.22 253–264 Exp 1470.61 Theor 1470.64	253–264 Exp 1470.61 Theor 1470.64

Lysine residues position within the OPN sequence are reported together with the identified peptides, their molecular weights expressed as as monoisotopic, and the experimental molecular weight obtained by MALDIMS or nano LC-MS/MS analyses.