Homomeric Proteins == Several structures of proteinaptamer complexes involving homomeric proteins have been reported over the years. as a consequence of the effective software of the cryo-electron microscopy technique to these systems. In the present paper, particular attention is definitely devoted to the articulated architectures that proteinaptamer complexes may show. Moreover, the molecular mechanism of the binding process was analyzed by collecting (Rac)-PT2399 all available information on the structural transitions that aptamers undergo, using their protein-unbound to the protein-bound state. The contribution of computational methods in this area is also highlighted. Keywords:aptamer, (Rac)-PT2399 crystal structure, X-ray crystallography, cryo-electron microscopy, NMR, proteinaptamer interface, molecular dynamics, allostery, ternary complex, protein data standard bank == 1. Intro == Intermolecular relationships represent key events in all biological processes. In living organisms, partnerships between biomolecules are characterized by high specificities and a wide range of binding affinities [1]. Proteins, key factors in all biochemical pathways, are promiscuous biomolecules whose activities generally rely on complex partnerships which they set up with many different chemical entities ranging from individual atoms/ions to huge macromolecules. With this scenario, it is not surprising the modulation of protein partnerships including those founded with other proteins represents a remarkable option in investigations aimed at developing fresh biomolecules of diagnostic and/or restorative interest [2,3,4]. However, the relationships that (Rac)-PT2399 proteins form with large biomolecules usually involve huge interfaces that cannot be efficiently inhibited with small molecules [5]. Consequently, it has been traditionally believed that antibodies could represent the obvious remedy to this issue [6]. However, the finding that DNA- or RNA-based polynucleotides endowed with the ability to specifically target proteins, including those not involved in relationships with nucleic acids in physiopathological conditions, could be developed with sensible costs has changed this perspective. Indeed, a wide range of proteins can be targeted by nucleic acids, denoted as aptamers, with affinities and specificities comparable to those exhibited by antibodies [7,8,9,10,11]. Aptamers are typically generated by using a process denoted as Systematic Development of Ligands by EXponential Enrichment (SELEX) in which randomly generated libraries of DNA or RNA sequences showing all possible bases in each position are exposed to the prospective [12,13,14,15,16,17]. Afterward, the sequences that do identify and bind the prospective are eluted and amplified by PCR and used in subsequent steps of selection of the strongest binders. Since the set-up of the SELEX approach three decades ago [18,19], thousands of different aptamers directed against proteins of restorative and/or diagnostic interest have been developed [10,14,20,21,22,23]. The many special properties of aptamers compared to protein-based therapeutics, which include the possibility of setting flexible designs, their rather straightforward production, and the opportunity to very easily improve them, possess generated a remarkable excitement for his or her potential to become effective biomarkers or medicines [7,24]. Although many aptamers have become tools of intense importance in fundamental technology [22,25,26,27,28,29,30,31], for many years only a single aptamer-based drug was in the marketplace, i.e., pegaptanib sodium (Macugen by Pfizer/Eyetech) that was authorized in 2004 from the FDA for macular degeneration [32]. Very recently, the FDA authorized PIK3R4 a second RNA aptamer, i.e., avacincaptad pegol (Izervay by Iveric Bio/Astellas) for geographic atrophy secondary to age-related macular degeneration [33]. Although this success may represent a turning point in the understanding of aptamers as attractive potential medicines, also considering their potential for treating acute conditions such as thrombolysis and cytokine launch syndrome [33], there is a obvious gap between the expectations and initial enthusiasm and the real outcome of so many investigations and tests [34]. There are of course many possible explanations for this so-called aptamer paradox [35]. Among others, the limited info currently available on aptamer structure and their mechanism of action represents a key point [34]. Indeed, despite the thousands of aptamers generated and characterized, a small fraction of them have been structurally investigated. Although the 1st structural characterization of a proteinaptamer complex was reported nearly three decades ago [36], limited progress has been made over the years. Initial analyses on proteinaptamer.