Molecular Interpretation of ACTH-b-Endorphin Coaggregation: Relevance to Secretory Granule Biogenesis Srivastav Ranganathan 1. , Pradeep K. Singh 1. , Uday Singh 2 , Praful S. Singru 2 , Ranjith Padinhateeri 1 *, Samir K. Maji 1 * 1 Department of Biosciences and Bioengineering, Wadhwani Research Centre for Biosciences and Bioengineering, Indian Institute of Technology Bombay, Mumbai, India, 2 School of Biological Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar, India Abstract Peptide/protein hormones could be stored as non-toxic amyloid-like structures in pituitary secretory granules. ACTH and b- endorphin are two of the important peptide hormones that get co-stored in the pituitary secretory granules. Here, we study molecular interactions between ACTH and b-endorphin and their colocalization in the form of amyloid aggregates. Although ACTH is known to be a part of ACTH-b-endorphin aggregate, ACTH alone cannot aggregate into amyloid under various plausible conditions. Using all atom molecular dynamics simulation we investigate the early molecular interaction events in the ACTH-b-endorphin system, b-endorphin-only system and ACTH-only system. We find that b-endorphin and ACTH formed an interacting unit, whereas negligible interactions were observed between ACTH molecules in ACTH-only system. Our data suggest that ACTH is not only involved in interaction with b-endorphin but also enhances the stability of mixed oligomers of the entire system. Citation: Ranganathan S, Singh PK, Singh U, Singru PS, Padinhateeri R, et al. (2012) Molecular Interpretation of ACTH-b-Endorphin Coaggregation: Relevance to Secretory Granule Biogenesis. PLoS ONE 7(3): e31924. doi:10.1371/journal.pone.0031924 Editor: Vladimir N. Uversky, University of South Florida College of Medicine, United States of America Received October 14, 2011; Accepted January 20, 2012; Published March 5, 2012 Copyright: ß 2012 Ranganathan et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by the Council of Scientific and Industrial Research (37(1404)/10/EMR-11, Department of Science and Technology (SR/FR/LS- 032/2009) and Department of Biotechnology, Government of India for financial support. The Funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: samirmaji@iitb.ac.in (SKM); ranjithp@iitb.ac.in (RP) . These authors contributed equally to this work. Introduction Amyloids are highly ordered protein/peptide aggregates that are associated with a number of human diseases including Alzheimer’s and Parkinson’s [1]. Natively structured or unstruc- tured peptides/proteins undergo structural transition to form cross-b-sheet rich amyloids. This transition is known to happen via partially folded intermediates and soluble oligomers [2]. Many proteins/peptides can form amyloids under certain environmental conditions suggesting that amyloid formation is a generic property of polypeptide chains [3]. Several studies in the recent past have found that amyloids are also involved in several native biological functions [4,5]. For example, amyloids of curli protein help E. coli for surface attachment and colonization [6]. Amyloids of chaplins are involved in aerial hyphae and fruiting body formation in filamentous fungi [7,8]. Moreover, yeast prions [9,10] and HET-s amyloid in Podospora anserina [11] are also well- studied examples of functional amyloids that are important for survival of the respective host and not just related to prevalence in diseased condition. The amyloid form of pmel17 in the melanosome appears to be used as a template for melanin synthesis and therefore protect melanocytes from reactive oxygen and free radicals [12]. In addition, aggregation has also been shown to be involved in secretory process of protein/peptide hormones [13]. Protein secretion in secretory cells of eukaryotes is primarily of two types: constitutive secretion and regulated secretion [14,15]. In constitutive secretion, the newly synthesized proteins are secreted via vesicular trafficking immediately after exiting the Golgi without any external stimulus. Whereas, in regulated secretion, secretory proteins/peptides are stored for extended periods of time in a highly concentrated form within membrane- enclosed structures (secretory granules) in the cytoplasm [16,17]. The secretory granules (SG) primarily consist of condensed aggregates of proteins/peptides [17,18]. Many secretory proteins have been shown to form intermolecular aggregates at granule relevant conditions in vitro, and protein aggregates were also observed in vivo. These observations suggest that reversible aggregation is the necessary step for condensing and sorting of the secretory granule proteins (for review see; [17,18,19]. However, it was not clear whether the aggregated and concen- trated proteins inside the granules are amorphous or contain any specific structure. It has recently been suggested that protein/ peptides are stored as amyloid-like structures inside the secretory granules of pituitary [13]. A large number of secretory protein/ peptide hormones formed non-toxic amyloids in vitro under granule relevant conditions. Also evident were amyloid like structures in the SG of hormones in AtT20 cells as well as rat pituitary tissue. However, the studies also showed that some of the peptide hormones such as ACTH and ghrelins do not form PLoS ONE | www.plosone.org 1 March 2012 | Volume 7 | Issue 3 | e31924