Abstract
Adiponectin, a collagenous hormone secreted abundantly from adipocytes, possesses potent antidiabetic and anti-inflammatory properties. Mediated by the conserved Cys(39) located in the variable region of the N terminus, the trimeric (low molecular weight (LMW)) adiponectin subunit assembles into different higher order complexes, e.g. hexamers (middle molecular weight (MMW)) and 12-18-mers (high molecular weight (HMW)), the latter being mostly responsible for the insulin-sensitizing activity of adiponectin. The endoplasmic reticulum (ER) chaperone ERp44 retains adiponectin in the early secretory compartment and tightly controls the oxidative state of Cys(39) and the oligomerization of adiponectin. Using cellular and in vitro assays, we show that ERp44 specifically recognizes the LMW and MMW forms but not the HMW form. Our binding assays with short peptide mimetics of adiponectin suggest that ERp44 intercepts and converts the pool of fully oxidized LMW and MMW adiponectin, but not the HMW form, into reduced trimeric precursors. These ERp44-bound precursors in the cis-Golgi may be transported back to the ER and released to enhance the population of adiponectin intermediates with appropriate oxidative state for HMW assembly, thereby underpinning the process of ERp44 quality control.
Highlights
ERp44 tightly controls adiponectin assembly in the early secretory compartment
In the immunoprecipitated complex milieu generated by using antiadiponectin antibody, two species were recognized by antiERp44 antibody (Fig. 1A), one migrating at ϳ130 kDa likely corresponding to the ERp44-adiponectin LMW complex and another migrating at around 190 kDa likely corresponding to the ERp44-adiponectin MMW complex
Far-Western blotting using purified recombinant adiponectin showed complexes with ERp44 corresponding to LMW and MMW adiponectin (Fig. 1B), but again no signal corresponding to the HMW form was detected (Fig. 1B)
Summary
ERp44 tightly controls adiponectin assembly in the early secretory compartment. Results: ERp44 exclusively recognizes and converts assembly-trapped adiponectin intermediates back to precursors of the biologically potent high molecular weight form. The endoplasmic reticulum (ER) chaperone ERp44 retains adiponectin in the early secretory compartment and tightly controls the oxidative state of Cys and the oligomerization of adiponectin. Our binding assays with short peptide mimetics of adiponectin suggest that ERp44 intercepts and converts the pool of fully oxidized LMW and MMW adiponectin, but not the HMW form, into reduced trimeric precursors. These ERp44-bound precursors in the cisGolgi may be transported back to the ER and released to enhance the population of adiponectin intermediates with appropriate oxidative state for HMW assembly, thereby underpinning the process of ERp44 quality control
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