Biochimica et Biophysica Acta, 772 (1984) 231-234 231 Elsevier BBA Report BBA 70155 FUSION OF SMALL UNILAMELLAR VESICLES INDUCED BY A SERUM ALBUMIN FRAGMENT OF MOLECULAR WEIGHT 9000 LENISE A.M. GARCIA, PEDRO S. ARAOJO and HERNAN CHAIMOVICH Departamento de Bioqulmica, Instituto de Qu'tmica da lISP, C.P. 20780, S~o Paulo, SP. (Brasil) (Received October 26th, 1983) (Revised manuscript received February 16th, 1984) Key words: Membrane fusion; Protein-membrane interaction; Serum albumin; Differential spectroscopy A peptide (P-9) comprising amino acids 307 to 385 of bovine serum albumin induced the fusion of small unilamellar vesicles of phosphatidylcholine at low pH. Upon acidification P-9 exhibited a ultraviolet differential spectrum characteristic of hydrophilic exposure of chromophores. This conformationai change, and the structure of P-9 composed of three amphiphilic helixes, suggested a general working hypothesis for the description of protein-induced membrane fusion. The understanding of the molecular features of membrane fusion, particularly in protein-depen- dent systems, is acquiring an increasing relevance since proteins play a central role in some fusion processes [1,2]. Protein-induced membrane fusion is being investigated in several model systems in- cluding Ca2÷ synexin [3], clathrin [4], viral protein [5] and albumin [6,7] induced fusion of vesicles. This latter system is characterized by the aggrega- tion and fusion of small unilamellar vesicles of phosphatidylcholine (PC) induced by the F con- formation of albumin [8] at low pH [6,7]. Using large fragments from pepsin cleavage of albumin (fragment P-31, M r = 31000; fragment P-35, M r = 35 000) we have recently shown that the nature of aggregates formed prior to fusion determines the size of the fusion products [9]. In this report we show that a peptide of M r 9000, obtained by pepsin degradation of serum albumin, induces the fusion of small unilamellar vesicles at low pH. This system allows the investigation, in molecular details, of the structure-function relationships in Abbreviation: PyPC, 2-(10-(1-pyrene)decanoyl)phosphati- dylcholine. protein-induced membrane fusion. Controlled pepsin treatment of albumin yields degradation products of molecular weights ranging from 6000 to 35000 (e.g. fragment P-31 (31000), fragment P-35 (35000), fragment P-16 (16000), fragment P-14 (14000), fragment P-9 (9000) and fragment P-6 (6000)) [10]. A low-molecular weight fragment was isolated from pepsin-treated bovine serum albumin by Sephadex G-100 gel filtration * Filtration through Sephadex G-50 gave a single peptide (Fig. 1) characterized as follows. Both Sephadex G-50 filtration and sodium dodecylsul- fate (SDS) polyacrylamide [11] electrophoresis in- dicated that the isolated peptide had a molecular weight lower than that of cytochrome c (Fig. 1). The extrapolated molecular weight was 9000 (Fig. 1A, inset). Proof of both purity and nature of the peptide was obtained by determination of the first three N-terminal amino acids. The N-terminal amino acid was determined by * Purified albumin was treated with pepsin in the presence of octanoic acid as described [9]. The fractions containing this fragment are those of the peak immediately following pool I of Fig. 1A in Ref. 9. 0005-2736/84/$03.00 © 1984 Elsevier Science Publishers B.V.