Table I.
Identification of carboxy-terminal phosphorylation sites of S6 isoforms by MALDI-TOF mass determination analysis of Ba(OH)2-treated trypsin-digested peptides
Peptide mass and sequence is provided for observed peptides with a mass value ± 1.0 D of the theoretical mass.
| S6 Isoform | Experimental Replicates | Peptide Mass (D) | Peptide Sequencea (No. of Phosphates in Peptide) | No. of Phosphates in Carboxy Terminus | Phosphorylation Status of Carboxy-Terminal Region RRS238KLS241AAAKAS247AAT250S251A-COOHa,b,c |
|---|---|---|---|---|---|
| S6 | 5 | 559.585 | LSAAAK(0) | 0 | SKLSAAAKASAATSA-COOH |
| 577.521 | ASAATSA(0) | ||||
| 774.496 | SKLSAAAK(0) | ||||
| 1,118.87 | LSAAAKASAATSA(0) | ||||
| 1,334.02 | SKLSAAAKASAATSA(0) | ||||
| S6a | 1 | 528.332 | RRSK(1) | 1 | SKLSAAAKASAATSA-COOH |
| 577.568 | ASAATSA(0) | ||||
| 1,315.77 | SKLSAAAKASAATSA(1) | ||||
| S6b | 3 | 528.806 | RRSK(1) | 2 | SKLSAAAKASAATSA-COOH |
| 542.808 | LSAAAK(1) | ||||
| 738.668 | SKLSAAAK(2) | ||||
| 1,298.04 | SKLSAAAKASAATSA(2) | ||||
| S6c | 3 | 527.247 | RRSK(1) | 3 | SKLSAAAKASAATSA-COOH |
| 543.117 | LSAAAK(1) | ||||
| 561.137 | ASAATSA(1) | ||||
| 739.342 | SKLSAAAK(2) | ||||
| 1,083.610 | LSAAAKASAATSA(2) | ||||
| S6d | 4 | 528.315 | RRSK(1) | 4 | SKLSAAAKASAATSA-COOH |
| 542.134 | ASAATSA(2) | ||||
| 542.134 | LSAAAK(1) | ||||
| 739.102 | SKLSAAAK(2) | ||||
| 1,066.31 | LSAAAKASAATSA(3) | ||||
| S6e | 6 | 528.651 | RRSK(1) | 5 | SKLSAAAKASAATSA-COOH |
| 1,245.53 | SKLSAAAKASAATSA(5) | ||||
| S6a′ | 7 | 546.136 | RRSK(0) | 3 | SKLSAAAKASAATSA-COOH |
| 542.362 | LSAAAK(1) | ||||
| 542.362 | ASAATSA(2) | ||||
| 913.404 | RSKLSAAAK(1) | ||||
| 1,279.93 | SKLSAAAKASAATSA(3) | ||||
| S6b′ | 2 | 547.123 | RRSK(0) | 3 | SKLSAAAKASAATSA-COOH |
| 542.277 | LSAAAK(1) | ||||
| 542.277 | ASAATSA(2) | ||||
| 757.147 | SKLSAAAK(1) | ||||
| 1,279.87 | SKLSAAAKASAATSA(3) | ||||
| 1,066.06 | LSAAAKASAATSA(3) | ||||
| S6c′ | 3 | 546.942 | RRSK(0) | 4 | SKLSAAAKASAATSA-COOH |
| 541.839 | LSAAAK(1) | ||||
| 1,048.46 | LSAAAKASAATSA(4) |
a
Bold type indicates phosphorylated residue
b
White box indicates phosphorylation at one of three possible residues
c
Shaded box indicates phosphorylation at two of three possible residues