public function addProtein(Protein $protein, $start = null, $end = null)
    {
        $entry = new ProteinEntry($protein);
        if (! is_null($start)) {
            $entry->setStart($start);
        }

        if (! is_null($end)) {
            $entry->setEnd($end);
        }

        $this->addProteinEntry($entry);
    }