Table 1.

List of PTMs observed with highest confidence in endogenous and recombinant β- and α-actin, based on the presence of b (forward) and y (backward) fragment ions encompassing the PTM in MS2 spectra and occupancy >5% in at least one of the actin samples

	β-Actin		α-Actin
PTM	Endogenous	Recombinant	Endogenous	Recombinant
Nt-acetylation	100 (61/61)	98 (96/98)	99 (200/203)	100 (258/259)
K18 acetylation	11 (7/61)	11 (11/98)	5 (10/203)	$4 (10/259)$
K50 acetylation	7 (16/217)	7 (16/231)	12 (42/337)	15 (53/353)
K61 methylation	6 (8/134)	8 (12/146)	$2 (4/217)$	5 (11/214)
K68 acetylation	19 (10/52)	16 (8/49)	17 (9/54)	9 (4/46)
H73 methylation	94 (246/263)	93 (286/308)	96 (412/430)	92 (314/341)
K84 acetylation	$1 (3/279)$	$1 (2/321)$	$3 (12/437)$	11 (40/361)
H101 methylation	10 (25/240)	10 (26/264)	6 (19/300)	5 (16/336)
K113 acetylation	$4 (9/240)$	$3 (8/264)$	5 (16/300)	5 (16/336)
H161 methylation	15 (57/372)	17 (68/410)	$4 (15/405)$	7 (33/501)
K191 acetylation	9 (21/224)	11 (31/284)	15 (74/493)	18 (125/706)
K213 acetylation	$4 (1/26)$	17 (5/30)	8 (5/65)	12 (18/146)
K215 acetylation	13 (6/47)	13 (7/52)	20 (24/119)	8 (14/176)
K215 methylation	13 (6/47)	12 (6/52)	5 (6/119)	5 (8/176)
Y218 phosphorylation	18 (32/179)	11 (22/201)	$4 (16/378)$	$1 (4/368)$
R256 methylation	7 (1/15)	8 (2/24)	7 (16/242)	6 (29/451)
H275 methylation	28 (13/47)	32 (26/82)	8 (52/664)	5 (45/965)
K284 acetylation	14 (8/59)	13 (11/82)	11 (46/419)	15 (63/420)
K284 methylation	14 (8/59)	9 (7/82)	$2 (10/419)$	$3 (13/420)$
K291 acetylation	17 (46/268)	20 (60/298)	21 (75/358)	21 (85/400)
K315 acetylation	$0 (0/4)$	7 (1/14)	30 (3/10)	$0 (0/12)$
K315 methylation	25 (1/4)	21 (3/14)	$0 (0/10)$	$0 (0/12)$
K328 acetylation	20 (2/10)	35 (6/17)	64 (14/22)	40 (6/15)
K328 methylation	20 (2/10)	12 (2/17)	$0 (0/22)$	$0 (0/15)$
H371 methylation	$4 (4/92)$	5 (5/110)	$2 (3/133)$	$4 (5/135)$

For each PTM, the percentile occupancy is defined as 100 × the ratio of the number of peptides containing the PTM to the total number of observed peptides (shown in parentheses). See also Fig. 4 and SI Appendix, Figs. S3 and S4.