Previous investigators have reported the presence of two dominant proteins, tectonin I (25 kDa) and tectonin II (39 kDa), in nuclei and nuclear matrix from plasmodia of Physarum polycephalum. We demonstrate, by a modification of the nuclear isolation protocol and by protease sensitivity, that the tectonins are not nuclear proteins but rather are located on the exterior surface of the plasma membrane.We report the sequences of cDNAs of tectonins I and II, which encode 217 and 353 amino acids, respectively. Tectonin I is homologous to the C-terminal two-thirds of tectonin II. Both proteins contain six tandem repeats that are each 33-37 amino acids in length and define a new consensus sequence. Homologous repeats are found in L-6, a bacterial lipopolysaccharide-binding lectin from horseshoe crab hemocytes. The repetitive sequences of the tectonins and L-6 are reminiscent of the WD repeats of the ␤-subunit of G proteins, suggesting that they form ␤-propeller domains. Tectonin II has an additional N-terminal domain that includes a 47-residue sequence highly similar to the galactoside-binding sequence of the B-chain of ricin. The tectonins may be lectins that function as part of a transmembrane signaling complex during phagocytosis.In its plasmodial form, the myxomycete Physarum polycephalum exists as a multinucleated syncytium that feeds on bacteria and organic detritus by phagocytosis. The many nuclei within a single plasmodium progress through the cell cycle synchronously, and at the end of the G 2 phase undergo closed mitosis. Because of these characteristics, several investigators have examined the P. polycephalum nuclear matrix (1-4) and reported that, as with mammalian nuclear matrix, the P. polycephalum matrix contained a number of proteins ranging from approximately 40 kDa to more than 100 kDa but that it differed from mammalian nuclear matrix by having two dominant proteins with reported molecular masses of 23-28 and 35-38 kDa as determined by SDS-PAGE.1 The same proteins have also been found associated with purified rDNA chromatin (5). We have termed these proteins tectonins I and II, respectively.In the present study we report the cloning and sequencing of the cDNAs for the tectonins, and we use trypsin digestion of cell fractions to assess their localization in the plasmodium. We find that the tectonins are not nuclear proteins but instead are located on the plasmodial surface, and we report a method for purifying P. polycephalum nuclei not contaminated with the tectonins.The tectonins share with lectin L-6 of horseshoe crab hemocytes (6) six repeats of a novel consensus sequence that may form a ␤-propeller structure. Additionally, tectonin II contains in its N-terminal region a sequence similar to the galactosebinding domain of the B-chain of the plant toxin ricin (7). The tectonins may share with lectin L-6 the ability to recognize the outer membrane lipopolysaccharide of Gram-negative bacteria for phagocytosis and utilize an additional affinity for galactose to expand the number of ligands that they re...