To study the cellular consequences of misfolding of plasma membrane proteins, a transmembrane model protein that was constitutively targeted to the plasma membrane was developed for use as a reporter molecule. The reporter consisted of a C-terminally truncated CD4, incorporating a flexible cytoplasmic linker (CD4tl), fused to the N-terminal DNA-binding domain of the wild type (wt) bacteriophage lambda repressor (CD4tl-lambda) or L57C mutant repressor (CD4tl-lambdaC). The CD4tl-lambdaC cytosolic domain was largely in native state at 26°C but predominantly nonnative state at 37°C thus allowing the use of thermal shifts to follow the fate of unfolded proteins. In this image, one of six of a group from Figure 1 of Pirjo et al., JCB 2010, the expression of the mutant plasma membrane reporter, CD4tl-lambdaC, at 37 degrees is seen with CD4 antibody (green) in a detergent-permeabilized COS7 cell. The intracellular processing of this nonnative, thermally unfolded protein nearly completely limits its targeting to the plasma membrane. This cell is co-labeled with an ER marker, calreticulin antibody (red).
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