Association of the chaperone alphaB-crystallin with titin in heart muscle.
alphaB-crystallin, a major component of the vertebrate lens, is a chaperone belonging to the family of small heat shock proteins. These proteins form oligomers that bind to partially unfolded substrates and prevent denaturation. alphaB-crystallin in cardiac muscle binds to myofibrils under conditions of ischemia, and previous work has shown that the protein binds to titin in the I-band of cardiac fibers (Golenhofen, N., Arbeiter, A., Koob, R., and Drenckhahn, D. (2002) J. Mol. Cell. Cardiol. 34, 309-319). This part of titin extends as muscles are stretched and is made up of immunoglobulin-like modules and two extensible regions (N2B and PEVK) that have no well defined secondary structure. We have followed the position of alphaB-crystallin in stretched cardiac fibers relative to a known part of the titin sequence. alphaB-crystallin bound to a discrete region of the I-band that moved away from the Z-disc as sarcomeres were extended. In the physiological range of sarcomere lengths, alphaB-crystallin bound in the position of the N2B region of titin, but not to PEVK. In overstretched myofibrils, it was also in the Ig region between N2B and the Z-disc. Binding between alphaB-crystallin and N2B was confirmed using recombinant titin fragments. The Ig domains in an eight-domain fragment were stabilized by alphaB-crystallin; atomic force microscopy showed that higher stretching forces were needed to unfold the domains in the presence of the chaperone. Reversible association with alphaB-crystallin would protect I-band titin from stress liable to cause domain unfolding until conditions are favorable for refolding to the native state.
Bullard
Belinda
B
European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany. bullard@embl.de
Ferguson
Charles
C
Minajeva
Ave
A
Leake
Mark C
MC
Gautel
Mathias
M
Labeit
Dietmar
D
Ding
Linlin
L
Labeit
Siegfried
S
Horwitz
Joseph
J
Leonard
Kevin R
KR
Linke
Wolfgang A
WA
D000818:Animals; D001665:Binding Sites; D002942:Circular Dichroism; D064211:Connectin; D004355:Drug Stability; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007136:Immunoglobulins / Q000737:chemistry; D008856:Microscopy, Fluorescence; D016253:Microscopy, Immunoelectron; D009124:Muscle Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009206:Myocardium / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D009210:Myofibrils / Q000378:metabolism; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017510:Protein Folding; D011494:Protein Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011817:Rabbits; D011994:Recombinant Proteins / Q000378:metabolism; D012518:Sarcomeres / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure; D038203:alpha-Crystallin B Chain / Q000032:analysis / Q000235:genetics / Q000378:metabolism*
D064211:Connectin; D007136:Immunoglobulins; D009124:Muscle Proteins; D010446:Peptide Fragments; D011994:Recombinant Proteins; C577285:TTN protein, human; D038203:alpha-Crystallin B Chain; D011494:Protein Kinases