The Organization of Cholesterol Esters in Membranes of Mycoplasma capricolum

Abstract

The organization of cholesterol esters in Mycoplusma cupricolum membranes was studied by differential scanning calorimetry. Cells grown in the presence of horse serum incorporated large amounts of cholesterol esters into their membranes. The cholesterolester-containing membranes after incubation at low temperature showed an endotherm characteristic of a cholesterol ester crystalline 4 isotropic liquid transition that was identical in membranes both before and after thermal protein denaturation. This transition was not observed in membranes of cells grown in medium in which the horse serum was replaced by bovine albumin, fatty acids and unesterified cholesterol unless cholesterol esters were added to the growth medium. In membrane preparations obtained both from cells grown in horse serum and from cells grown with bovine albumin plus cholesterol and fatty acids, the free cholesterol content was sufficient to eliminate the bilayer order/disorder transition observed in isolated membrane phospholipids. Our studies indicate that the majority of cholesterol esters in M . cupricolum membranes is not present in attached serum lipoprotein particles, nor is intimately associated with membrane protein, but exists as relatively large cholesterol ester droplets or pockets tightly associated with the membrane. The cholesterol esters in these pockets appear relatively pure, although the presence of small amounts of other membrane components is likely.Cholesterol esters do not commonly form a significant component of biological membranes. In nature they are found in substantial amounts in serum lipoproteins [I] and atherosclerotic lesions [2]. Cholesterol esters have a low solubility of about 5 mo1/100 mol in lipid bilayers and when present in excess of this form a separate phase [2]. This phase may exist in various physical states depending on such factors as environment, composition, and thermal history [3,4]. The low solubility of cholesterol esters relative to that of free cholesterol in bilayers [5 -71 is probably related to the absence of a free 3-[j-hydroxyl group and the necessity for the bilayer to accommodate both a long acyl chain and a sterol ring in the proposed horse-shoe configuration [8]. This configuration places both the cholesterol moiety and its acyl chain parallel to the acyl chains of the bilayer phospholipids and has the ester group near the hydrophilic interface.The cell membranes of Mycoplasmu provide an exceptional membrane system with regard to sterols. Unique among procaryotes, most Mjmplasmas spp. require free cholesterol for growth [9]. N o Mycopplusma sp., however, has been found capable of synthesizing cholesterol [lo]. The Mycoplasma take free cholesterol from the growth medium and incorporate it unaltered into the plasma membrane [II, 121. Free cholesterol is an efficient regulator of bulk membrane fluidity helping to maintain the cell membrane in a homogeneous physical state despite variations in temperature and the fatty acid composition of the growth medium [9,12,13]. In many Mycoplasma spe...

The Phospholipid Profile of Mycoplasmas

“…When grown in a serum supplemented growth medium, the main sterol found in mycoplasmas is unesterified cholesterol, despite the presence of excessive amounts of esterified cholesterol in medium. The low levels of esterified cholesterol incorporated from the growth medium is not required for growth and appears to form lipid droplets or pockets in the membrane [11]. …”

From hot to cold: dissecting lipidome adaptation inMycoplasma mycoidesand the Minimal Cell JCVI-Syn3B

“…We next compared the lipidomes of M. mycoides and Syn3B across all temperatures to visualize how much they remodel their lipid composition during temperature adaptation (Fig.2b). This time we considered all lipid classes except for cholesterol-ester which is taken up, but enriched within the cell, and not in the membrane 2829 (and Supplementary Figure 1b)). The distribution of samples shows that both organisms remodel their lipidomes at different temperatures, however, in different ways.…”