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A Complex Dance: The Importance of Glycosaminoglycans and Zinc in the Aggregation of Human Prolactin

Christensen, Line Friis Bakmann, Malmos, Kirsten Gade, Christiansen, Gunna, Otzen, Daniel Erik
Biochemistry 2016 v.55 no.26 pp. 3674-3684
binding sites, denaturation, glycosaminoglycans, humans, neutralization, pH, prolactin, secretory granules, thermal stability, zinc
The zinc binding hormone pituitary human prolactin (hPRL) is stored in secretory granules of specialized cells in an aggregated form. Glycosaminoglycans (GAGs) are anionic polysaccharides commonly associated with secretory granules, indicating their involvement in granule formation. Here we, for the first time, study the impact of GAGs in combination with Zn²⁺ on the reversible hPRL aggregation across the pH range of 7.4–5.5. Zn²⁺ alone causes hPRL aggregation at pH 7.4, while aggregation between pH 7.4 and 5.5 requires both Zn²⁺ and GAGs. GAGs alone cause hPRL aggregation below pH 5.5. Comprehensive thermal stability investigations show that hPRL is particularly destabilized toward thermal denaturation at pH 5.5 and that GAGs increasingly destabilize hPRL at decreasing pH values. We propose that Zn²⁺ causes hPRL aggregation through low-affinity Zn²⁺ binding sites on hPRL with GAGs facilitating Zn²⁺ binding by neutralizing repulsive positive charges of hPRL in the acidic environments of the TGN and mature secretory granules. In a manner independent of the aggregation-causing agent(s), the different hPRL aggregates show very similar secondary structure and amorphous morphology. We speculate that this may be a recognizable sorting signal in the formation of hPRL granular vesicles.