Titin: quantitative mass measurements by scanning transmission electron microscopy and structural implications for...

Scanning transmission electron microscopy has been used to investigate mass and linear mass density of native titin-2, a large soluble fragment of intact titin, from rabbit skeletal muscle. Dark field images of unstained, freeze-dried titin-2 appeared as either compact globules or looser and larger balls of string. Direct mass measurements indicated that the compact forms have an average mass of 2.40 +/- 0.50 x 10(6) Da. The mass to length ratio, determined from well-spread portions of titin strands (3-5 nm wide) from the ball of string forms, averaged 2.7 +/- 0.9 kDa/nm. Thus a single native intact titin molecule has a calculated contour length of well above approximately 1 micron, sufficient to span unidirectionally between the Z line and M line region in a resting-length sarcomere.