The following statements are made with respect to the major classes of heat shock proteins (HSPs) in plants:
A. Cytosolic HSP100 is essential while the chloroplast HSP100 / ClpB family protein is not essential for heat stress response.
B. Proteins of the HSP90 family are exclusively localized in the nucleus.
C. Members of the HSP60 protein family (chaperonins) are abundant even at normal temperatures.
D. The C-terminal domain of small HSPs is homologous to α-crystallins, proteins found in the vertebrate eye lens.
Which one of the following options is a combination of all correct statements?

Correct option is D

Correct Answer:
Option (4) – C and D
Explanation:
C is correct: HSP60 family proteins (chaperonins such as GroEL homologs) are housekeeping chaperones and are constitutively expressed, even under non-stress conditions.
D is correct: Small HSPs contain a conserved α-crystallin domain, homologous to vertebrate eye lens α-crystallins.
A is incorrect: Both cytosolic HSP100 and chloroplast-localized ClpB/HSP100 proteins play important roles in thermotolerance; chloroplast HSP100 is also functionally significant.
B is incorrect: HSP90 proteins are found in the cytosol, nucleus, and other compartments, not exclusively in the nucleus.
Information Booster :
· Heat shock proteins function as molecular chaperones to prevent protein aggregation.
· Small HSPs act early during stress by binding unfolded proteins.
· HSP60 and HSP70 families operate during normal cellular metabolism.
· Subcellular localization of HSPs is diverse and stress-dependent.
Additional Information (Incorrect Options):
Options 1 and 3: Include statement A, which is incorrect.
Option 2: Includes statement B, which falsely restricts HSP90 localization.