Light harvesting complexes 2 (LH2) are the peripheral antenna proteins in the bacterial photosynthetic apparatus and are built of alpha/beta-heterodimers containing three bacteriochlorophylls and two carotenoids each. Previously, we have found in 2D-crystals that the complexes could be inserted within the membrane with a tilt with respect to the membrane plane (Rhodobacter sphaeroides) or without tilt (Rubrivivax gelatinosus). To investigate whether the tilted insertion represents the native state or if it is due to specific 2D-crystal contacts, we have used atomic force microscopy to investigate LH2 from Rhodopseudomonas acidophila reconstituted at different lipid to protein ratios. High-resolution topographs could be acquired of two types of 2D-crystals or of densely packed membranes. Interestingly, in type 2 2D-crystals and in non-crystalline densely packed membranes. cylinders are integrated with their symmetry axis normal to the membrane plane, while in type 1 2D-crystals LH2 cylinders are integrated with a tilt of similar to4degrees with respect to the membrane plane. Therefore, we present strong evidence that the tilt of LH2 does not represent the native membrane state and is due to protein-protein contacts in specific 2D-crystals. (C) 2004 Elsevier Inc. All rights reserved.