PT - JOURNAL ARTICLE AU - Francisco Javier Lobo-Cabrera TI - A Constant Proportion in Protein Structure AID - 10.1101/500025 DP - 2018 Jan 01 TA - bioRxiv PG - 500025 4099 - http://biorxiv.org/content/early/2018/12/19/500025.short 4100 - http://biorxiv.org/content/early/2018/12/19/500025.full AB - The principles governing protein structure are largely unknown. Here, a structural proportion universal (R2 = 0.978) among proteins is reported. The model variance is shown to be independent from protein size, secondary structure composition, compactness or relative surface area. The structural characteristic under study --named here QUILLO-- quantifies residue-type spatial clustering. In this way, polar, hydrophobic, acidic and basic residues are evaluated individually and their values added up. For the analysis, all X-Ray currently determined structures deposited in the Protein Data Bank were studied. The QUILLO proportion offers for the first time an a priori protein prediction quality-check. Indeed, predictions with unexpected proportion values correspond to low ranks in the CASP12 experiment. The reason behind a specific, constant rule for protein folding remains unknown.