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Similarly to epidermal growth factor (EGF) and EGF-like repeats, the "P-domain" represents a cysteine-rich module that has been detected in the past in a variety of polypeptides, as well as in high molecular weight proteins. Here, a precursor for a secretory polypeptide (xP2) is characterized that consists of two P-domains. xP2 has been discovered in Xenopus laevis with the help of the polymerase chain reaction. In contrast to all other P-domain peptides, it is synthesized in the skin but not in the stomach or the pancreas. By this and other criteria, it cannot be considered simply as the X. laevis homologue of recently described P-domain peptides, viz. the spasmolytic polypeptides (PSP/hSP/mSP). Furthermore, a polyclonal antiserum was generated against the deduced C-terminal end of xP2. Due to its immunoreactivity with granular glands, as well as with the epidermis, the possibility of a growth factor activity for xP2 in the germinal layer is discussed.
FIG. 1. Nucleotide sequence and translation of the xP2 transcript as deduced from the cDNA clones pxP2-5’-5.1 (positions
1-116), pxP2-1.1 (positions 39-278), and pxP2-3‘-4.2 (positions 201-5460). The potential cleavage site for signal peptidase is
indicated by an arrow. The conserved tryptophan residue in each P-domain is encircled; the polyadenylation signal and restriction sites are
underlined. Also marked are the positions of the synthetic oligonucleotides. The sequence selected for the synthetic peptide (SKP-2) is
indicated bv a dotted line. The triawle between Dositions 99 and 100 denotes the insertion observed in the APEG protein (Gmachl et al., v 1990).
FIG. 2. SDS-polyacrylamide gel electrophoresis (15%) and
subsequent Western analysis of SDS-extracts from X. laevis
skin. Lane a, staining with antiserum SKP-2; lane b, staining with
preimmune serum; lanes c and d, staining with antiserum SKP-2
after preadsorption with peptide SKP-2 (lane c) or peptide XGP-1
(lane d ) .
FIG. 3. SDS-polyacrylamide gel electrophoresis (15%) and
subsequent Western analysis. Lanes a and b, mucus fraction of X.
laevis skin (same individual as in lanes c and d); lanes c and e,
sonicated extracts of X. laevis skin from two different animals (other
individuals as investigated in Fig. 2). Lanes d and f, SDS-extracts of
X. laeuis skin from the same two different animals. In lane a, strong
staining with the FIM-A.l antiserum (SPL-5; Hauser et al. (1990))
is shown, whereas lanes b-f represent the reaction with antiserum
SKP-2.
FIG. 4. SDS-polyacrylamide gel electrophoresis (15%) and
subsequent Western analysis of SDS-extracts from X. lueuis
skin (lunes u and d )a nd of homogenate6 from pancre(alsu nes
b and e) and stomach (lunes c and P). All samples originate from
a single animal, and comparable amounts of total protein (about 5
pg) were loaded onto each lane. In lanes a-c, relatively weak staining
with antiserum SKP-2 is shown, whereas lanes d-f represent the
strong reaction with the xP4 antiserum (XGP-1; Hauser and Hoffmann
(1991)).
FIG. 5. Immunohistochemical analysis of X. laevis skin. A and C, positive staining (brown) of the epidermis (e), granular glands (g), and regenerating granular glands (arrows) with antiserum SKP-2. Mucous glands (m) show no immunoreactivity. B, no signals were obtained after staining with the preimmune serum. Scale bars are 100 pm A and B) and 50 pm (C).
FIG. 6. Comparison of P-domains compiled from all P-domain peptides identified so far in X. laevis (xP1 and xP4, Hauser
and Hoffmann (1991); xP2). Invariant amino acids are enclosed in bones. The 6 characteristic cysteine residues are numbered. The stars
in the deduced consensus sequence for X. &vis P-domain peptides denote positions that are not conserved in P-domain peptides from other
species (pS2, hSP, mSP, and PSP).
