CaBP (15 kDa Ca²⁺- binding protein)

Function

The 15 kDa protein (CaBP) is the most abundant low molecular weight Ca²⁺- binding protein, different from calmodulin, in eggs of sea urchin, Hemicentrotus pulcherrimus.
This protein belongs to the troponin C superfamily.
The 15 kDa protein plays a key role in the regulation of cell division. This protein probably functions as Ca²⁺- dependent intracellular modulator of microtubule assembly (Hosoya et al., 1988).

Protein

CaBP is the 15 kDa protein belonging to the troponin C superfamily. This protein is composed of 150 amino acid residues including two Trp residues and one Cys residue. The molecular weight was calculated to be 16,739, supposing the N-terminus was blocked with acetyl-group (Hosoya et al., 1988).
SWISS_PROT: P21788

Subcellular location

Immunofluorescent microscopy experiments showed that the distribution of the 15 kDa protein in the cell changes during mitotic cycle.
In unfertilized eggs fluorescence was strongest in the nucleus. Cytoplasm seemed to fluoresce evenly. When fertilized eggs or eggs at mitosis were stained with antibody, prominent labeling of the nuclear region or the mitotic apparatus (MA) throughout mitosis was observed. At 45 min post-insemination fluorescence in the nuclear region of the eggs revealed short fibrous structures radiating from nucleus. At 60 min post insemination, eggs entered the streak stage. The 15 kDa protein redistributed along the "streak" and the staining formed the "eye-blow" like structure. Fluoescence in the nucleus seemed to become stronger.
At metaphase and telophase eggs 70 and 90 min after insemination, respectively, immunoreactivity was distributed throughout the spindle and aster, but not obviously restricted to any particular class of microtubules within the spindle. The 15-kDa protein appeared to localize almost exclusively in the MA region from metaphase through telophase. The cleavage furrow was not stained (Hosoya et al., 1988).

Expression Pattern

Imunofluorescent and immunomicroscopic observations showed that the 15 kDa protein is localized in the nuclei of fertilized eggs and mitotic apparatus of dividing eggs (Hosoya et al., 1988).

Spatial localization
Method 1: Immunofluorescent assay
Reference: Hosoya et al., 1988
Method 2: Immunomicroscopic assay
Reference: Hosoya et al., 1988

Stage Unfertilized eggs Fertilized eggs Dividing eggs

Tissue fluorescence is strongest in the nucleus, cytoplasm staines evenly nuclei mitotic apparatus

Ectopic expression

Antibodies microinjection

The affinity purified anti-15 kDa protein antibody was microinjected into one of the blastomeres at two cell stage 10 min after first cleavage. Microinjection resulted in the arrest of cell division of the injected blastomere.
Microinjection experiments support the hypotesis that the 15 kDa protein plays a key role in the regulation of cell division (Hosoya et al., 1988).

Sequences

GenBank

Regulatory Regions

Regions

Regulatory Connections

Upstream Genes

CaBP

Downstream Genes

Evolutionary Homologues

Spec1
CaM H. sapiens

Bibliography

UrchiNet

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Stage	Unfertilized eggs	Fertilized eggs	Dividing eggs
Tissue	fluorescence is strongest in the nucleus, cytoplasm staines evenly	nuclei	mitotic apparatus

CaBP (15 kDa Ca2+- binding protein)