I am looking for interactions between a protein (ACTB) and calcium and I want to use the paxtoolsr package. Is this possible? What code is necessary?
Here is code that would get you interactions that include actin or calcium from Pathway Commons:

    library(paxtoolsr)

    # READ AND FILTER PATHWAY COMMONS SIF INTERACTION TABLE ----
    # With the file PathwayCommons12.reactome.hgnc.txt.gz downloaded from 
    # https://www.pathwaycommons.org/archives/PC2/v12/ and uncompressed 
    # NOTE: Either download directly or use downloadPc2() 

    # Read file 
    dat <- readSifnx("PathwayCommons12.reactome.hgnc.txt")

    # Pathway Commons uses CHEBI identifiers for small molecules, chemicals, drugs, metabolites, etc. 
    # IDs: 
    # * Calcium: "CHEBI:29108" from https://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:29108
    # * Actins from https://www.genenames.org/data/genegroup/#!/group/929

    ca2 <- "CHEBI:29108"
    actins <- c("ACTA1", "ACTA2", "ACTB", "ACTBL2", "ACTC1", "ACTG1", "ACTG2")

    # Subset the SIF interaction table to those entries with either calcium or actin
    # Use idsBothParticipants=TRUE if it must be both source and target nodes must come from ids parameter
    sif <- filterSif(dat$edges, ids=c(ca2, actins), idsBothParticipants=FALSE)
    sif_ids_both <- filterSif(dat$edges, ids=c(ca2, actins), idsBothParticipants=TRUE)

    write.table(sif, "calcium_actin_interactions.txt", sep="\t", quote=FALSE, row.names=FALSE)

    # > head(sif)
    # # A tibble: 6 × 7
    # PARTICIPANT_A INTERACTION_TYPE PARTICIPANT_B INTERACTION_DATA_SO… INTERACTION_PUB… PATHWAY_NAMES MEDIATOR_IDS
    # <chr>         <chr>            <chr>         <chr>                <chr>            <chr>         <chr>       
    # 1 ACTA2         in-complex-with  CALD1         Reactome             NA               Smooth Muscl… http://path…
    # 2 ACTA2         in-complex-with  ITGA1         Reactome             NA               Smooth Muscl… http://path…
    # 3 ACTA2         in-complex-with  ITGB5         Reactome             NA               Smooth Muscl… http://path…
    # 4 ACTA2         in-complex-with  LMOD1         Reactome             NA               Smooth Muscl… http://path…
    # 5 ACTA2         in-complex-with  MYH11         Reactome             NA               Smooth Muscl… http://path…
    # 6 ACTA2         in-complex-with  MYL10         Reactome             NA               Smooth Muscl… http://path…
