Abstract
Pulmonary surfactant protein C (SP-C) is a small, extremely hydrophobic peptide with a highly conservative primary structure. The protein is characterized by two adjacent palmitoylated cysteine residues, two positively charged residues (one arginine residue and one lysine residue) in the N-terminal region, and a long hydrophobic stretch. SP-C enhances the adsorption of phospholipids into an air-water interface. To determine the importance of the positively charged residues, we carried out experiments with natural porcine SP-C and modified porcine SP-C (SP-Cm) in which the positive charges had been blocked by phenylglyoxal. Circular dichroism experiments showed that SP-Cm had an increased content of alpha-helix. Natural SP-C, but not SP-Cm, catalyzed insertion of phospholipids into a monolayer at the airwater interface. This reduced insertion was due to a strong reduction of binding of phospholipid vesicles to the monolayer. The insertion catalyzed by the natural porcine SP-C was decreased by an increased pH of the subphase. In contrast to natural SP-C, SP-Cm induced lipid mixing between phospholipid vesicles. The extent of lipid mixing was a function of the SP-C content. We conclude that the positively charged residues of SP-C are important for the binding of phospholipid vesicles to the monolayer, a process that precedes the insertion of phospholipids into the monolayer.
Highlights
From the sl.aboratory of Veterinary Biochemistry and §Centre for Biomembranes and Lipid Enzymology, Utrecht University, 3508 TD Utrecht, The Netherlands
The modified surfactant protein (SP)-C exhibited a peak at 45 min, whereas the natural surfactant protein C (SP-C) fraction came off the column at 37 min (Fig. 1)
Circular dichroism measurements showed a difference in secondary structure between the natural porcine SP-C and SP-Cm collected from the interface (Table I)
Summary
(Received for publication, August 29, 1994, and in revised form, February 9, 1995). Lambert A. Essential for breathing, is a complex mixture of lipids and proteins It is present at the air-liquid interface of the lung, and its main function is to stabilize the lung by reducing the surface tension. SP-C, when present in phospholipid vesicles [18, 19] or monolayers [15,20], was able to catalyze the insertion of phospholipids into a monolayer or to induce the lipid mixing between neutral phospholipid vesicles and pyrene-PC-Iabeled vesicles [15] It altered the thermodynamic properties of membranes [11, 21]. To elucidate the importance of the positively charged amino acids for the function or structure of the SP-C, we used the reaction of phenylglyoxal with arginine residues [22, 23]. The experiments show that these residues are important to bind vesicles prior to the insertion of phospholipids from the vesicles into the monolayer
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